>However, even the dimensions for finished lumber of a given nominal size have changed over time. In 1910, a typical finished 1-inch (25 mm) board was 13⁄16 in (21 mm). In 1928, that was reduced by 4%, and yet again by 4% in 1956. In 1961, at a meeting in Scottsdale, Arizona, the Committee on Grade Simplification and Standardization agreed to what is now the current U.S. standard: in part, the dressed size of a 1-inch (nominal) board was fixed at 3⁄4 inch; while the dressed size of 2 inch (nominal) lumber was reduced from 1 5⁄8 inch to the current 1 1⁄2 inch.[11]
Despite the change from unfinished rough cut to more dimensionally stable, dried and finished lumber, the sizes are at least standardized by NIST. Still a funny observation!
Funny you say that, because "two by fours" used to be 2" x 4”, but became progressively thinner as manufacturing processes improved.