IEEE TRANSACTIONS ON PROFESSIONAL COMMUNICATION, VOL. PC-22, NO. 2, JUNE 1979 93

The U.S. Patent Classification System KENDALL J. DOOD

Abstract-The heart o f the s y s t e m ut i l ized by t h e U.S. Patent and Trademark Off ice for the storage and retrieval o f technical in format ion is a hierarchical s c h e m e for the c lass i f icat ion o f pa ten t s based o n the technica l subject mat ter d i sc losed in t h e m . Mult iple c o p i e s o f p a t e n t s are s tored in the s y s t e m according t o their c lass i f icat ions , and technica l in format ion is retrieved from the s y s t e m b y de l ineat ing f ields o f search in terms o f c lassif icat ions m o s t apt t o inc lude all d i sc losures relevant t o the subject matter t o be retrieved.

PATENT documents consist essentially of descriptions of practical solutions to technical problems. Since the aim

of engineering is to find practical solutions to technical prob­lems, patent documents constitute an important engineering resource. Like other technical publications, however, the extent' to which patents are used is sources of technical in­formation is related not only to their technical content, but also to the ease with which those of particular interest can be selected and retrieved from all the rest. These factors-thf content, selectability, and retrievability of technical docfi* ments -are all part of the subject of professional communica­tion. The selectability and retrievability of patents in particular and how these factors are provided for in the patent collections of the United States Patent and Trademark Office are the sub­jects of this paper.

The U.S. Patent and Trademark Office (PTO), in its head­quarters in Arlington, Virginia, maintains two identical files of the more than four million unique patent documents it has published in the last 142 years. One of the files is reserved for use by employees of the PTO; the other is open to the public. The employees of the PTO who use its file most extensively are the approximately 1000 patent examiners who search the file for the technical information they need to determine the patentability of inventions for which patent protection has been requested. In order for them to search the file in a rea­sonable amount of time, some means must be available by which they can locate those patent documents most pertinent to the subjects of their searches without having to look at all the patents in the file. The means provided is the U.S. Patent Classification System. This system is also available to the pub­lic who may use it j o search the PTO's public file. An under­standing of how this system works is therefore of practical interest to anyone who may wish to use the patent file as a source of technical information.

Manuscript received October 1 2 , 1 9 7 8 ; revised February 6 , 1 9 7 9 . T h e author is a Supervisory Patent Classifier in the U.S. D e p a r t m e n t

o f C o m m e r c e Patent and Trademark Of f i ce , Wash ington , D C 2 0 2 3 1 , ( 7 0 3 ) 5 5 7 - 1 7 5 0 .

THE U.S. PATENT CLASSIFICATION SYSTEM

In the U.S. Patent Classification System (PCS), all patented technology is divided into about 350 classes of subject matter which are, in turn, subdivided into approximately 100,000 subclasses: Each class and subclass is uniquely numbered and each is titled. (The PTO continually revises its classification system, adding new classes and deleting old ones so that at any time there may be gaps in the class-number sequencer) A par­tial list of the subclass titles of a typical class, Class 358, "Pictorial Communication: Television," appears-in Fig. 1. Each class and subclass also has a "def ini t ion"-an expanded description of the particular subject matter denoted by its title including, in many cases, "search notes" or references to other classes or subclasses containing related subject matter. Fig. 2 shows a portion of the class definition of Class 358, and Fig. 3 shows a portion of the Class 358 subclass definitions.

The position of a subclass within a class is an important feature of the system. Listings such as the one in Fig. 1 in which the subclasses are arranged in their proper positions rela­tive to one another are called "class schedules." In the class schedules, as can be seen in the figure / the titles of some of the subclasses are indented with respect to the others. Subclasses 2 to 4, for example, are indented under subclass 1, and sub­class 5 is indented under subclass 4. All subclasses whose titles begin in vertical alignment are said to be "coordinate" With respect t o one another. Subclasses 1, 75, 8 1 , and 83 are co­ordinate subclasses. Coordinate subclasses such as these whose titles begin farthest to the left (not indented at all) are called "mainline" subclasses and serve as the headings of the primary divisions of the subject matter of the class; they are always printed in upper case type. Coordinate subclasses such as 2 to 4 , 10 to 12, 17, 2 1 , 4 1 , and 56 which are indented one level to the right under a mainline title represent a secondary division of the subject matter* of the primary division; coordi­nate subclasses such as subclasses 13 to 15 indented under sub­class 12 represent a division of the subject matter of the sec­ondary division, and so on. Thus, the subject matter of an indented subclass is denoted not only.by its own title but by the titles of each of the succession of subclasses under which it is indented. The. complete title of subclass 25, for example, should be understood to be Natural color tetévision: specific signal processing circuitry, modulator of demodulator: refer­ence signal control or source. This implicit narrowing-down-by-indentation of the scope of the subject matter of indented subclasses is always made explicit in the corresponding, sub­class definitions. This can be seen in the definition of subclass

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96 IEEE TRANSACTIONS ON PROFESSIONAL COMMUNICATION. VOL. PC-22, NO. 2 , JUNE 1979

May 1 977 35H-1 CLASS 358. PICTORIAL COMMUNICATION. TE l £ VISION

Original Classif icat ion 1974 5 M Elet trolun.nii «-ηΐ . r s-.lid s tate bO . Pro jec t ion de ν ice

1 NATURAL COLOR TELEVISION 61 Using b i r e l r imien l 01 p o i a n / t n t ' 2 . Holography medium 3 . Stereoscopic f)2 Using delormable medium 4 Recording o r reproducing tvi U i im ' me. h a m . a i - >pli. ai s, annmg 5 Using dil iracl ion techniques ur stripe m e a n s onlv

f i lters 64 Lsinii cathode rav tube Isintt photographic re.t ord h 1 - un M j w i J l t e i i . f ; 11 I h m ! ' s,, a

' 7 , With co lor signal in nonpu t o n a l ii-t m H . Correct ing reco ide r ·..·! r e p r - x l u c . .· l -.nu the ^ait t « » , ! 1 · !>*• λϊ·· : >t

r r r s i r s • m m , ' than ..ne < ορ , ι 9 Separa te ly ret ordVd pi > η ι . ι η > .rtvi tj7 'Ait! tu-an, ρ si ' i . in .ndn atinn r .» aris

signais f s . H<:ri/.onU»l Vtt ijxvs , 10 Monitoring; testing, or measu i mi; », » Ph-.t.- t. ·: ' ·

11 Signal format ·. om «· ι su-ii r 70 • S e t .m-lar'v . . » m t s s i < » n ^ t j s . . | -12 . Transmiss ion s ignal formatting 71 'ΛΐΙ ί . e s * . ' : ".-.(,!!, α . ,r ^, î , , î ; ï 13 . Pul.se c < *ie modulation 7 ? With l>» j.ïi < m u:\ il» ter m mint . >Lo 14 . StmuJUne-ou* airô s ^ u e i i t i a i stgjnal* 73 Vlr ItJ i î ·.< t i a 0 ί t U-ptî, il p v n r l i a t . -r. 15 . Simultaneous s ignais >i t'u • tr >n 1* .m. tôt. tii» .un îmes - ' 16 v , . . Luminance plus duai-phast rnx iu iau ι c e n t laver *

color c a r i i e t 74 R ta ' i li.tt > , M t , ι ,jit 1 -uppls 1 m'rol 17 Color sviu hi m t i a t . .·. 7 5 Ν M l RAI t Ol CM* f- \CMMII h 18 Of line-*M?quentia 1 switch 7 h C · t >* ΐ.ιρίιν· , ι ϊ · . u λ", ι -19 . Using b u r s t » 77 . S i /e \ a i tat ion _ • 2 0 , . Specific Imrst separation >·ιη vit! 7.1 lntcrmedi.it»-» - . t o r a c e -,A s l cna t -2V S pe C l i i c s ignaï ρ r t. *· e s m i » ν, c 11 ν • a ι ! r ν 7>. H:.u f -„*i.a. - . M i l . >2 Special elle» ts M< 1 ('-.{..t . •a-rectt.vff 23 , Modulator or . te imviuut- ι *l PhEl I M m ( ) I (»H 24 . . . . For PAL M g n a l -, V. il) ·>γλι nai ν Ί V u .( ί - ι r, it-, » 1· e : 2 S . · . . Refe rence s ignai .••.rfiiioi . 0 . h , . u n e TE LEV 151 ON 26 . . Color Killer ; 84 Au-.ltrn.-r s - j r u ; a n d u s e a'-countiiu. 27 . • Color s ignal ampl i tude γ·«μι ν ι *5 i ·' '< 1 * 1 1 - U ,> ν iv. .f. - \ u . .111 ί jj' | i .c J ' / ' -2» Hue con t ro l W î · ; l u .a 1. i - i iiii an 1 d i -M . t» i i -29 . Color balance or t em|?era tur t . ;

«7 l'an, : a.-r.i. 30 . · Matnxmg o i m i x i m . St. η s. , „ 31 . . C h r u i n u u n c e - luminance s i g n a i s e pat at η .η · ~* » PseU.t.- ;

.42 . G a m m a . orre."tj..»n y p . Η· .Lit'! aphte 33 H tank 11 it \ •<i s* pa i . i t » \ t< -v «,>.(! s ame l.anm 34 , . U.C. i n se r t ion *J2 A Ht : ru le 1. ' \ . u i M t n ' i i 1 35 . Differential phase <>* ampl i tude , d i s tor t ι·.··; < '? 1·, i. .u a. ' . . r t 1·,. tal< d l-\ a s .

correct ion j 94 Ai, : : t U.» t. , .ne 36 Nvuse e l imination ^5 . lia K s. a t t e : ifdii» t r •:, 37 Signal enhancuiti · ' «t Contour n e n e r a t o r 38 . Frequency response niodiluation ' '.•7 ί i. t>na- 1 t. n". ι-!·»! and η . ί ι - . γ . lur.

' 39 Luminance c h a n w i ι irtuM.·ν j . . he. . ! : - , m,* 40 . . .Chrominance channel c i r c u i t r y . ' ! l. ;ç,!ci a a t e r Ί1 P i c t u r e s ignal g e n e r a t o r s ; l oo H a / a : * ' -u- ; ma , t

42 . With i'.nA mg 1 oU.t ι lit*-1 s l-"l . Manuiai !uniit . 43 . . With one p ick -up device supplying m i t ; 1 0 J i 1 m i .s. r .ti-1 K - l i i t - u .

than:une co lo r signai ' | „ » Ή •Navita'tioil 44 . . . Single pho tosens i t ive device \ ÎU4 Mi;iulat. : 45 With indexing \ 10 2 M 'tu ' Ai. t e . t, 1 46 Including conclu» dive gr id at t a rçe t j 100 F :,t-x u tet t : 47 . Frequency separable s ignals 107 . Measurement 4B . . . Color s ignals ACstfparale output' f 10* . . S u r v e m a m - e .

terminals ' ! lu « . Aerial view inn 4 it-. . . . . Separate complete i m a g e s on lace of I 110 Hi. >-\ « s u . · m ,ι>λ i-ibl» »'.« u".

photosensitive de vu e . | 111 . . X-rav 50 . . With more Ui'an one' p ick-up device , each ; 112 . ϊ> ·λϊ ι , : oj u i t rasut i ic

supply in* only 'one color signa! j 113 , Luira red 51 . . . Misregis trat ion c o r r e c t i o n j 114 M e a n - ' - ρ ;* n j n t ujiautht 1 m il asi .1 sjt;ru'i 52 . · . Having lour photosensitive d e u c e * j 115

M e a n - ' - ρ ;* n j n t ujiautht 1 m il asi .1 sjt;ru'i

53 . . Using'mechanical-optical scanning m e a n s ! 116 . Marque* or barra-r onlv ; 117 . L'or relation tesl tnu

54 Motion picture tilni scanning j 11» .. . Mask t m; s ignai 35 . Op t i c - . j l l ' j Ν t n s U n d a i d s . an sequt in»-5.6 .' P i c t u r e uispiav i 120 i ν ne pu1se a t te ra t ton

' 57 R e d - w h u e phenomena f 121 Audio scrambling 5* . . With moi ing color l i l ters ! 122 . . C-1inii

Fig. 1 . First half o f the class schedule for "te levis ion. '

25 in Fig. 3 which refers to the definition of subclass 23 which, in turn, refers to that of subclass 21 and so on back to the definition of subclass 1 (not shown) which refers back to the definition of 4he class. All current class schedules are collected in a volume titled Manual of Classification, which is available in the public search room. Class and subclass definitions as­sociated with each schedule are available there ajso.

C l a s s i f y i n g P a t e n t s i n t o t h e S y s t e m

The classification of a patent in the system is based on the most comprehensive claim in the patent. The claims are that part of the patent document which formally specifies what exactly is new and is protected by the patent grant. The

patent claims, therefore, are significant from both a technical and a legal point of view. The most comprehensive claim in a patent is the one which encompasses more of the disclosed technical subject matter than any other; this claim is used, for classification purposes, as a summary of the whole technical disclosure.

Patents are classified by examiners and classifiers of the ETO who are experienced in the subject matter of the patents involved and in the rules of classification. In placing a patent into the system, the most appropriate class is selected first. Then, the mainline subclasses of the selected class are con­sidered, and the first of these, starting from the beginning of the class, that describes at least some of the technical subject "matter of the most comprehensive claim is selected. The co-

DOOD: PATENT CLASSIFICATION SYSTEM 9 7

DATE March 1974 CLASSIFICATION DEFINITIONS

CLASS 3 5 8 , PICTORIAL COMMUNICATION. TELEVISION

Page 358-1

I GENERAL STATEMENT OF THE CLASS SUBJECT MATTER

A This class deals with the transmission of pic­tures and o r their transient o r permanent reproduction either locally or remotely, bv methods involvmg the following steps

Step (1) The scanning of a picture, ι e. , the analysis of the whole of a pic­ture-containing area by resolving it into an area or a plurality of a r eas of which at least one of the two d i m e n ­sions is elemental, simultaneously o r in a sequence.and the derivation in response thereto of a p ic iure- repre­sentative electric signal.

Step (2) The reproduction of the whole of a picture-containing area by the repro­duction in response to a picture-rep­resentative electric signal of an area oT a plurality of a reas of which at least one of the two dimensions is ele­mental, simultaneously or in at sequence

B. In this c lass Pictorial Communication Sys­tems are those systems for the transmission and reproduction of arbi trar i ly composed'pic­tures in which the local light variations c o m p o s ­ing a picture are not subject to variation with time, e . g . , documents <twith.jtTitten and printed', maps .char t s , photographs wither than m o t i o n picture film*

C. In this class Television Systems are those systems for the transmission and reproduc­tion υ{ arbitrari ly composed "pictures in which the local light variations composing a picture may change with time, e.g. . natural "live" scenes, recordings of such scenes.

II SUBCOMBINATIONS OF PICTORIAL COMMUNICA­TION OR TELEVISION SYSTEMS

A. This class includes circuits specially de­signed, for dealing with television or pictorial communication signals, as distinct from merely signals of a particular frequency range.

Β Electrical circuits not specific to pictorial communication or television whicn may con­stitute subcombinations of such apparatus are classified m the appropriate c lass for such circuits.

Ill COMBINATIONS OF OTHER APPARATUS WHICH INCLUDE APPARATUS OF THIS CLASS

A. This class does not include the following subject matter:

1. Systems in which legible alphanumeric or like character forms are analysed ac­cording to Step (1) to derive an electric signal from which the character i s recog­nized by comparison with stored informa­tion, which are dealt with in C lad's 340, subclass 146. 3.

2. Systems for the direct photographic copy­ing of an original picture in which an elec­tric signal representative of a characteristic of the picture is derived and employed to mod π ν the operation of the system, which are dealt with in Class 355, subclass 38.

. Systems for the reproduction according to Step 12) of pictures comprising alpha­numeric or like character forms but in­volving the production of the equivalent of a signal which would be derived by Step ! 1), e . g . , by cams, punched card or tape, coded control signal or other means, which are dealt with in Class 340, subclass 324.

IV RELATED ELEMENTS OR SYSTEMS POUND ΓΝ OTHER CLASSES

35, Education, subclass 8 » , teaching devices including television combinations.

90, Gear Cutting, Milling and Planing, sub­classes 13* and 24.4, pattern controlled devices including scanning.

96. Photographic Chemistry. Process and Materials , various subclasses, particularly subclass 1*. pictorial information recording.

179, Telephony, subclass 100.1 *, sound re ­cording and reproducing.

204. Chemistry, Electrical and Wave Energy, subclass 2, electroprinting.

250. Radiant Energy, subclasses 200* and 216-, photo-cells, circuits and optics there­for.

307. Electrical Transmission or Interconnec­tion Systems, various subclasses, particu­larly subclass 200 ' , non-linear solid state circuits .

312. .Supports, Cabinet Structures, subclass 7. television cabinets.

313. Electric Lamp and Discharge Devices, subclass 364», cathode ray tubes.

315, Electric Lamp and Discharge Devices, Systems, subclass cathode ray*tube circuits .

317, Electricity, subclass 1 2 4 p h o t o s e n s i ­tive device in circuits for electromagnets.

318, Electricity, Motive Power Systems, sub­classes 41 and 85, synchronization circuitry.

324, Electricity, Measuring and Testing, sub­class 77, analysis of complex waves.

325. Modulated Car r ie r Wave Communication Systems, various subclasses, t ransmit ters and receivers .

328, Miscellaneous Electron Space Discharge Device Systems, ^ I M O U S subclasses, part ic­ularly 1 0 8 1 2 1 ^and 178*. pulse circuitry, storage systems and circuitry with partic­ular wave output.

329, Demodulators and Detectors, subclasses 50 and 1 0 4 s y n c h r o n o u s and pulse demod­ulation.

330, Amplifiers,- subclass 11, D.C. re inser­tion circuit.

Çig. 2 . Def in i t ions f o i t h e classif icat ion "te levis ion. 1

ordinate subclasses indented one space to the right under this subclass are then considered, and the first-rccurring one of these which further describes the subject matter of the most comprehensive claim is selected. The selection process is re­peated for the next level of indented coordinate subclasses, if any, and so on, until there is no appropriate subclass remaining.

In placing a patent pertaining to color-killer circuitry into Class 358, for example, subclass 1 would be selected from the mainline subclasses as being the first one to describe the sub­

ject matter of the patent, i.e., color television. Then subclass 21 would be selected as being the first in its set of coordinate subclasses to further describe lite subject matter of the patent —specific signal processing circuitry. And, finally, subclass 26 would be selected as being the first .in its set of coordinate subclasses to further describe the subject matter of the patent -color-killer circuitry. (Exceptions to rthis general procedure for placing patents occur, but where they do, notice is given in the definition of the affected class or subclass.)

98 IEEE TRANSACTIONS ON PROFESSIONAL COMMUNICATION, VOL. PC 22 , NO. 2, JUNE 1979

Subject matter under subclass 1 including specific clrquitry peculiar to natural color television but not restr icted in usability to any single system, signal generator or display device specifically provided for elsewhere in this class .

1> Note Examples of subject matter found herein are specific local oscillator circuitry and ajnphtude control of ref­erence wave.

22. Subject matter under subclass 21 including appa­ratus for modifying the signal by the selective insertion or deletioi\of signal portlSns, or r e ­placement by further signals.

(1) Note Fading and chroma keying are exam­ples of effects produced by devices clas­sified herein.

« SEARCH CLASS 352, Optics, Motion Pictures; subclass 85, for

special effects in motion picture photogra­phy

23. Subject matter under subclass 21 particularly adapted for modulating onto or deriving from a car r ie r or subcarr ier a component of a natural color television signal.

SEARCH CLASS . 325, Modulated Carr ie r Wave Communication

Systems, appropriate subclasses, for modu­lator-demodulator systems.

329, Demodulators and Detec&8*, appropriate subclasses, for demodulators and detectors.

332, Modulators, appropriate subclasses, for modulators.

24. Subject matter under subclass 23 of particular utility in modulating or demodulating a car r ier whose phase is alternated at line rate .

25. Subject matter under subclass 23 including a particular source of or control for a reference signal.

SEARCH CLASS 331. Oscil lators, appropriate subclasses,

for osci l lators and control means therefor. »

26 Subject matter under subclass 21 including apparatus which disables the chrominance channel upon occurrence of a non-color signal or a chrominance signal with insufficient amplitude or phase coherence.

Subject matter under subclass 21 including ^ apparatus for controlling the amplitude of a color information signal.

SEARCH CLASS: 333, Wave Transmission Lines and ne tworks ,

subclasses 14-18, for amplitude controlled transmission systems.

28. Subject matter under subclass 21 including apparatus, for changing the phase of the chrominance signal and, or reference signal to adjust the hue of the displayed signal.

SEARCH CLASS 307. Elec t r ica lTransmiss ion or Interconnec­

tion Systems, subclass 262. for non-linear solid state phase controlled circuits.

328. Miscellaneous Electron Space Discharge Device Systems, subclass 155, for electron rube pPh^e correction systems.

Fig. 3 . Partial list o f subclass de f in i t ions for " te lev i s ion ."

The last subclass selected according to this process becomes the "original" classification of the patent. A copy of the patent is labeled with the class and subclass numbers and stored in the file in patent serial number order with other patents bearing the same classification. This copy of the patent is referred to as the "original" copy. A file of patents arranged in this manner is referred to as a "classified" file. Both files in the PTO are classified files.

Cross-Referencing

Since in classifying original copies into the system one goes no farther into the class than the first-occurring appropriate co­ordinate subclass at successive levels of indentation, the danger exists that more patents will be placed into the earlier-occurring subclasses than into later ones. To avoid this, the order in which subclasses are arranged and the concepts which form their bases are given very careful consideration when schedules are created. There are many instances, however, when the ar­rangement of subclasses, no matter how carefully thought out, and the rules governing the placement of patents into these subclasses, no matter how carefully followed, still do not adequately provide for all the significant technical information contained in the patents. For example, a patent pertaining to color television whose most comprehensive claim encompasses a detailed description of a combination of color-killer circuitry and hue-control circuitry would, according to the rules of clas­sification, be placed as an original copy in Class 358, subclass 26 since it is the first-occurring subclass descriptive of at least some of the subject matter of the patent-color-killer circuitry in this case-in the ultimate set of coordinate subclasses. The

information in this patent pertaining to hue-control circuitry would thus appear to be lost even though subclass 28 specifi­cally provides for this subject matter. However, when a sub­class exists which is appropriate for significant information not specifically provided for by the original classification of a patent, the patent rrfay be given an additional "cross-reference" classification. Another copy of the patent, designated as a cross-reference copy, is marked with the class and subclass numbers of the cross-reference classification (Class 358, sub­class 28, in our example) and stored in patent number sequence with the other patent copies-originals and cross-references-marked for that subclass.

RETRIEVING TECHNICAL INFORMATION

FROM THE 5>YSTEM

There are a number of ways in which t e c h n i c information can be retrieved from classified files. One is by looking up the classifications of patents known to be relevant and then find­ing other patents in the same subclasses. Another is by look­ing up the serial numbers and classifications of patents granted to specific inventors or assignedhto specific assignees in the various indexes of such bibliographic information maintained by the PTO.. But the Patent Classification System is designed primarily to facilitate searches for patents relevant to a partic­ular technical subject when no other information about the patents is known.

Delineating the Field of Search

Various listings of the classes appear at the beginning of the Manual of Classification, The most convenient of these for the

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purpose of subject matter searching is the one entitled "Patent Office Classes Arranged By Related Subjects." It is usually-possible by scanning this list to pick out one or«two classes that appear likeTy to contain the subject of the search. Sug­gested classes (and subclasses) can also be found by looking up terms relevant to the search subject in another PTO publica­tion titled Index to the U.S. Patent Gassificatian. Reviewing the definitions of the classes arrtved'at in either of these ways is usually sufficient to confirm the 'selection of one as the most appropriate class for further investigation.

The subject matter of c l a ^ s , unlike that of subclasses, is-mutually exclusive. Therefore, the searcher can expect not to have to look for patents pertaining to exactly the same subject matter in more than one class. Subcombinations of that sub­ject matter may, however, be classified in,another class. Simi­larly, combinations of the subject matter with that of another class may be classified in the.other class, and<species of the subject matter may be classified in still other classes. Cathode ray tubes, for example, of the type used in 4he color television systems of Class 358 are classified per se in Class 313 , "Elec­tric Lamp and Discharge Devices," while color television sys­tems which are parts of teaching systems are found in Class 3,5, "Education."^ Thus, the searcher must be aware not only of what the subjects of tKè classes are, but also what the scopes of the classes are relative to their subject matter. All this in­formation is contained *m the-definitions of the classes; see, for example, the references to Classes 35 and 313 in Part IV of "the "Class 358 definitions in Fig. 2. Deducing this informa­tion from the class titles alone is usually impossible.

Once the most likely ciass is selected, the class schedule is consulted and the maintint'subclasses, are scanned for the first-occurring one which explicitly provides for some feature of

the subject matter of the search. This and the subsequent steps are the same as in the process ol(placing a patent into the system.

If, for example, you were searching for color television hue-control circuits, you would scan the mainline subclasses of Class 358 and select subclass 1 as the first-occurring one de­scriptive of the subject matter of your search. Scanning the

, coordinate subclasses indented one space, to the right under subclass I. you would select subclass 21 , "Specific signal pro­

cessing c i rcu i t ry" as the next subclass that further describes some feature of the subject of your search, and scanning the coordinate-subclasses indented one space to the right under this subclass, you would finally select subclass 28. You have now defined the limit of your field of search.

Because of thé way patents are classified into the classes, you can be sure that no original copies of patents pertaining to hue-cqntrol circuitry appear lower in the schedule. How­ever, original copies of patents pertaining to hue-control cir­cuitry may appear in any of the preceding subclasses either in combination with or .as part of the subject matter specifically described by those subclass titles. Therefore, the inclusion of 'these subclasses in the field of search must be considered.

Fortunately, in practice, not all preceding subclasses need to be included in the field of search. Usually, in fact, relatively few of them will b e - a t least on the first search trial. One reason is that schedules are created so that the subject matter of mainline subclasses and of other coordinate subclasses at low levels of indentation tends to be mutually exclusive as often as passible. Second, the nature of the subject matter of many superior subclasses is so specialized that even though subject matter pertaining to the search is not specifically ex­cluded from them, finding it in them described in any signifi-

too IEEE TRANSACTIONS ON PROFESSIONAL COMMUNICATION, VOL. PC 22 . NO. 2, JUNE 1979

cant detail is unlikely. Finally, where earlier-occurring sub­classes do appear likely to contain relevant information, all such information is likely to be found also in the limit sub­class in cross-reference cop ies / We saw in our example of cross-referencing practice^for instance ,\that a cross-reference copy of a patent in Class 358 pertaining to the combination of color-killer circuitry and hue-control circuitry Would appear in subclass 28, the limit subclass in our search example, even though the original copy is classified in subclass 26.

Other factors to be considered in delineating or expanding a field of search include suggestions for further searching which may appear in the search notes appended to the definitions of the class and subclasses relevant to the search. The search notes of subclass 28, for example, suggest other places to search for similar circuitry whege the nature of the signal itself is not a restricting factor. Also, you may choose to add to your field of search subcfasses that are designated as the original classifi­cations of particularly relevant cross-reference copies found in the limit subclass or % other subclasses in your initial search field. As a rule, however, it 's best to start with a small, man­ageable search field consisting of the most likely locations of relevant information while keeping in mind other possibilities if the results of the first trial aren't satisfactory.

How extensive the field of search ultimately becomes de­pends on the judgement of the searcher. Most searchers.famil-iar with the system are satisfied with their results after only a few adjustments ..to the field. Even if they have retrieved no relevant document at that point, they are confident that, al­though there may be some relevant information somewhere in the system accessible through continued adjustments, the chance of the information's being significant enough to war­rant the'possibly great additional effort involved in ^retrieving it is nil. Searchers unfamiliar with the system, on the other hand; shoièU^allow for more adjustments in the beginning, but they can expect to be able to reduce the number necessary to satisfy themselves after very little experience with the system, especially if the subject matter of their searches is confined to a small number of classes.

APPENDIX

U.S. PATENT CLASSIFICATION SYSTEM PUBLICATIONS

The set of publications designed to aid in the use of the U.S. Patent Classification System is extremely important. The Patent and Trademark Office provides and maintains on a cur­rent basis a number of highly effective and relatively easy to

use publications concerning the PCS. By combined, systematic use of these, publications, it is possible to locate patents in the search file. The basic Patent and Trademark Office publica­tions are the Index to U.S. Patent Classification, the Manual of Classification, and the Classification Definitions. The Index and the Manual are distributed and sold to the public by the Government Printing Office. a Currently, Definitions is sold to the public in more limited quantities by the Patent and Trademark Office.

• The Index to U.S. Patent Classification comprises an exten­sive alphabetical listing of subject matter headings or descrip­tors based on the system used by the Patent and Trademark Office to classify .patents. The Index serves ,as an initial means of entry into the system by providing the user with specific citations of class(es) and subclass(es) relevant to his particular area of interest. It includes both tech­nical and common terms that are designed to encompass all matter patentable under our patent laws, e.g., machines, manufactures, processes, and compositions of matter^

• The, Manual of Classification is required fof locating specific information in the U.S. patent file. It is maintained in as current a condition as practicable and, as a result of the con­tinual revision and reclassification activities, generally reflects the contemporary state of any given art. The primary feature of the Manual is its inclusion of class listings, referred to as class schedules, for each class of invention. Each sched­ule lists, in an ordered outline form, every subclass included in a class, identifying each by a short title.

• The Classification Definitions is the third essential PCS tool. The Definitions comprises a series of publications containing more than 10,000 pages which give a detailed definition for each class and subclass title in the Manual. The Def­initions is essential in that the short subclass titles found in the Manual are not always indicative of the subject matter to be found in, or excluded frorn>fca subclass. It has been written to indicate the scope of the interpretation to ,be given to the words in a subclass title. By limiting or expand­ing in a precise manner the meaning intended by the Patent and Trademark Office for each subclass title, guidance is pro­vided t o that , universally, users of theManual will be directed to the same subclass for particular patents by eliminating, insofar as possible, varying and subjective interprétations of subclass titles.