What is Four-Square?

Craftsmanship depends on tight fitting, accurate joinery. Without observing the fundamental steps in basic stock preparation, this is impossible. That is why it is necessary to mill the boards we use four-square before cutting joints or shaping them. So what does four-square mean?

A board that is four-square is perfectly straight and flat, where each side is at right angles (90 degrees) to its adjacent sides. Why is this important? If the boards you use are not uniform in shape, then the joints you cut will be sloppy. Any sloppiness in one chair leg will be magnified by the inaccuracies of the apron pieces it is joined to. When the entire chair frame is assembled, no glue will be strong enough to keep the joints from racking, eventually breaking the bond. The shear stresses on the glue lines will be too great. On the other hand, if the joints fit snuggly, and there are no gaps where the shoulder of the tenon on the chair legs meet the apron, then the joint will hold the chair together and the glue bond will not undergo undue stress.

The chief cause of inaccuracy in stock preparation is a failure to establish generalized reference points. Yes, your tablesaw fence may be a few thousandths out of adjustment, if you are picky, or your jointer fence may be off a half a degree, but these transgressions can be mitigated by measuring, marking, cutting, planing or performing any other action using the same corner as your starting point. This corner is bound by the face side and face edge of the board.