The perfect timber already inside your crooked stick

Nominal minus ½ in. That is the whole trick of square rule.

Your timber comes off a portable mill. It is green, it is crooked, a 7×7 measures 7 1/8 here and 6 5/8 there. You cannot lay out a joint from a face that moves.

Square rule settles it in one sentence: assume a PERFECT timber, smaller, contained inside the real one. Nominal minus ½ in. A 7×7 becomes 6½×6½. An 8×8 becomes 7½×7½. All the joinery refers to that timber, never to what was delivered.

What you gain: like pieces are interchangeable, and you can lay out a joint before its mate is even sawn. What you pay: a housing to cut in the mortised piece, and a reduction on the tenoned one — carried 1½ in back of the shoulder, finished with a 45° chamfer.

Direct consequence for your dimensions: a 12 ft sill measures 12 ft minus 15 in shoulder to shoulder. Not 12 ft. And the shoulder-to-shoulder figure governs everything — a short tenon does not change your building's size; bad arithmetic does.

Source : beemer-timber-frame-square-rule.md §4