Nominal size

The name a piece of softwood lumber is sold under, such as 2 by 4 or 1 by 6, which is larger than its actual dressed dimensions.

A 2 by 4 measures about 1.5 by 3.5 inches, a 1 by 6 measures about 0.75 by 5.5 inches, and the gap grows with the size of the board. Every plan that gives dimensions to cut from must say which convention it uses, because the difference is significant: on a 2 by 4 it is nearly a third of the width.