This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The mechanic who attempts to fasten a mitered frame in the home workshop usually comes to grief. This is due to lack of proper facilities for holding the frame straight and out of wind, and for clamping or drawing the miter joint together after the glue has been applied. The little device shown in the sketch, if properly made and used, does away with the usual difficulties and annoyances. It consists of a triangular block of wood with raised strips on the two edges that make the right angle, and the clamping piece with the wood screw through the center on the long side. The raised strips are made somewhat thinner than the frame to be fastened, as the clamping piece should bear on the frame and not on the strips; the function of the strips being to hold the frame square. The triangular block should be large enough to take the corner of the frame and leave room enough for the wood screw that holds it in place on the block. Four of these blocks will be necessary and they should be used in conjunction with pinch dogs. These dogs come in different sizes and may be purchased at supply stores or made as shown. The outside of the legs should be straight and parallel and the inside tapered so as to draw the joint together.
Ill: Clamp for Holding the Corner of a Frame While Gluing and Fastening the Mitered Joint
When a joint is ready to be glued, a piece of paper is placed on the block under the joint to keep it from sticking. Apply the glue and push the two sides into the corner formed by the raised strips, the dog is then driven in lightly and the clamping piece screwed down tightly, and if the miter has been properly cut, a nice close-jointed and square corner will be the result. - Contributed by J. Shelly, Brooklyn, New York.