Locate the middle of the length of the lower piece, called Y, on one arris, and from this point lay off on this arris half the width of the upper piece, X.From this point square across Y with the knife and try-square.

Lay X again in its place, exactly along the line just scored.

Then mark with the knife on Y the width of X, which may then be removed and the second line squared across Y.From these two lines square across both edges of Y to approximately one-half the thickness.
Now turn X face down, lay Y on it, and mark it in the same way as Y.
Set the gage at one-half the thickness of the pieces, and gage between the lines on the edges, taking care to hold the head of the gage against the marked faces.

Then even if one piece is gaged so as to be cut a little too deep, the other will be gaged so as to be cut proportionately less, and the joint will fit.
Cut a slight triangular groove on the waste side of the knife-marks, Fig.

91, p.