The Key to a Proper Fit for Keyways

Did you ever have someone tell you that you cannot fit a square peg into a round hole? This common phrase is misleading because it details the dimensions of neither the peg nor the hole. Clearly if the peg is 1-inch square and the hole is 4 inches in diameter, then the peg will easily fit in the hole, albeit useless. However, if that same peg was 25 mm square with a tolerance 0/-0.01mm on each side and the hole was Ø25mm with a tolerance of +0.01/0mm, it would slide in easily and grab the hole at the corners.