R301.8

Section R301.8 is titled 'Nominal sizes' and covers the requirements for the intended application and usage convention of lumber dimensions used throughout this code. This section establishes that nominal dimensions are being used throughout this code unless otherwise specifically noted. The purpose of this section is to prevent misinterpretation and misapplication of this code.

The model shows an actual size 2x4 being planed and sanded down to its final nominal dimensions (which is 1-1/2" x 3-1/2"). There are many resources available on the internet that explain the history of how nominal lumber dimension usage came into common practice in the construction industry.