Molten metal will shrink as it cools. To allow for this, patterns are made slightly oversized. A pattern that is 12 1/8" long will produce a 12" long part when cast in iron. Shrink rules are elongated rulers that take shrinkage into account. (A one foot shrink rule with 1/8" shrink per foot looks like an ordinary ruler. It is actually 12 1/8" long.) By using a shrink rule to measure parts for a pattern, the pattern maker does not have to scale up the dimensions of the desired casting.