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.

And along the length, you lay out from one end. Square one end of the timber: that is your 0. Every dimension is taken overall from that end, face by face, and you hatch what comes off. Then you cut the joints.