The great table
1x: 3/4" actual. 2x: 1.5". Widths: "2" = 1.5, "4" = 3.5, "6" = 5.5, "8" = 7.25, "10" = 9.25, "12" = 11.25. The width shrink grows with nominal size because drying and surfacing remove a constant from a smaller planed face. Memorize the 2x row and the 1x row; the rest is on the tag anyway.
Why the lie exists
Lumber is named at the sawmill (rough, green, full dimension) and then shrinks: kiln-drying removes moisture, planing surfaces removes roughness. The name stuck from the mill stage; the sticker reflects what the mill sold. Every trade learns the mapping once; homeowners meet it at the first stubborn fit.
Using actual sizes in design
Cabinets, shelving, and built-ins must design in ACTUAL dimensions (a "2-foot" shelf of 2x12 lumber is 23.25" wide nominally — but 11.25" deep, not 12). Framing design uses nominal (that is what spans tables and yards speak). Knowing which world you are in prevents the 1/4-inch surprise that ruins the fit.