The name on the lumber rack describes a board that no longer exists: nominal sizes refer to the rough-sawn green dimensions, and by the time the board has been dried to 19% moisture content and surfaced four sides, the American Softwood Lumber Standard (US DOC PS 20) fixes a smaller actual size. The reductions follow a system — nominal 1 in thickness dresses to 3/4, nominal 2 to 1-1/2, nominal 4 to 3-1/2; widths through nominal 6 lose 1/2 in while nominal 8 and wider lose 3/4 — which is why a 2x6 is 5-1/2 in wide but a 2x8 is 7-1/4, not 7-1/2.
Timbers play by their own rule: stock nominal 5 in and thicker is ordinarily sawn and sold green, dressing 1/2 in off each face, so a 6x6 measures 5-1/2 in square and an 8x8 sells at 7-1/2 in square. Metric labels follow the actual dressed size at 25.4 mm per inch — the familiar 38 × 89 mm stamped on a 2x4.
The commercial twist is that purchasing math ignores the shrinkage entirely: lumber is still bought by the board foot computed on nominal dimensions, so a 2x6 counts as 2 by 6 at the register even though the tape measure disagrees. Layout and joinery must use the actual size; the invoice never does.