What Is Shrink Rule/Pattern Makers Scale?

Shrink rule or pattern makers scale is a special (oversized) rule used for easiness of pattern making. Different shrink rules are used for different material.  The dimension mark on a shrink ruler has actually included the proportionate shrink allowance for that material. This oversized scale compensates shrinkage allowance, so one can measure the dimension as ordinary ruler without worrying the contraction of the metal.

Every material has a specific shrinkage value. For example, when iron is the cast material, a pattern that has a size 12 1/8 inch long produces the 12-inch long cast. In this case shrink rule of 1 foot (12inch) for iron is actually  1/8 inch longer.
Shrinkage rates for some common metals are given below

Iron 1/8" per foot
Brass 3/16" per foot
Aluminum 3/16" per foot
Copper 3/16" per foot
Steel 1/4" per foot
Lead 5/16" per foot
Zinc 5/16" per foot
Tin 1/12" per foot