Slips and Skids: An explanation of the terms is necessary

http://www.empire-aviation.com/flight-instructors/john-e-mclain/slips-and-skids-an-explanation-of-the-terms-is-necessary.html

There is a much easier method to determine the bank angle required for a coordinated standard rate turn. Take your airspeed, drop the last digit, and add five if your are using knots and seven if your are using miles per hour. Thus, if your speed is 130 knots, add five to 13 and you come up with 18 degrees of bank required to maintain a standard rate turn. Try 120 mph. You should come up with 19 degrees of bank. Believe me, up to about 250 knots, these values are accurate enough for all practical purposes.

To understand the skid better, use the turn coordinator again. While flying with the wings level, apply some rudder, in our example here, left rudder. The airplane will turn left, and eventually bank left. What you now want to do is prevent the bank by applying opposite aileron to keep the wings level. Now you will be turning with the wings level, but surprise, the little airplane in the turn coordinator will be deflected to the left, the direction of the turn, even though the wings are level. This again shows that the turn coordinator does not accurately depict the position of the wings of the big airplane unless the ball is centered.