Nominal Size

Nominal size is the labeled, traditional name for a piece of dimensional lumber -- a "2x4" or "1x6" -- which no longer matches its actual finished dimensions. A construction 2x4 is milled down to roughly 1.5" x 3.5" after drying and surfacing; the "2x4" name refers to its rough size before that processing, a naming convention that's stuck around long after the actual dimensions changed. Always check actual measured dimensions, never the nominal label, before cutting a joint or a fit that depends on the real number.