On this page you can see one of the ways to construct a dovetail bridle joint (corner through dovetail joint).
Wherever oblique cuts are to be made, great care is necessary in marking. Place the pieces in the position shown in Fig. 1; the upper piece is to have the tenon, or dovetail, the lower the mortise. With try-square and sharp pencil, mark lines around three sides of each piece, at a distance from the end equal to the width of the opposite piece, as shown at a, Fig. 1. These pencil-marks should be very light, so as to be easily cleaned off with sandpaper or smoothing plane.
Set the gauge at W, and mark the lines bb, Fig. 2. Set it at Q, and mark the lines b’b’. Set it at X, and mark the line cc, and press the point only of the gauge at d, d. Set the gauge at Y, and mark the line c'c', and the points e, e. Bring the edge of the blade of the try square to coincide with the lines b and c on the end of the mortise piece, and mark with the knife a line joining them. Do the same for all the oblique lines, as shown in Fig. 1.
The tenon or dovetail (a, Fig. 2) is sawed out, and the sides of the mortise b also cut with the saw. The mortise is finished with the chisel. A vertical cut is made as at a, using the mallet, then one at b; these to be repeated until one half through the piece, then cut on the opposite side. Avoid cutting into the sides of the mortise by inclining the chisel. The same caution must be observed in keeping some of the wood at c, until the last, when it is carefully cut away, and the surface tested with the try square.
The sides of the mortise usually need a little paring before the tenon will fit. This done, the pieces should go together easily, but without play or open joints.
The joint ought to go together by light driving, and be perfectly square on the inside between the working faces. If it is found to be satisfactory, take it apart, apply a light coating of glue, and drive together again. When the glue is hard, the joint may be sandpapered and squared.