The speed square serves a variety of uses.
Sometimes called a “magic square,” angle square, or protractor square, this tool functions as a square but is shaped like a triangle. One leg of the triangle has dimensions marked on its face in inches; the other has a raised ridge on the top and bottom to allow it to be butted to the workpiece. The third and longest side of the right triangle, the hypotenuse, has degrees (zero to ninety) marked on it to help in measuring and marking miter cuts.
Made of cast aluminum, the speed square serves most of the same purposes as the try square: You can use it to check a cut or joint for square, to mark cutoff lines, or as a straightedge to identify warped or cupped boards. The magic square is also handy as a cutting guide when using a hand-held circular saw.
The speed square is available in two sizes. The smaller size is seven inches on a side (the hypotenuse is just under ten inches), while the larger version is twelve by twelve by seventeen inches (actually, for sticklers who know the Pythagorean Theorem, the precise measurement is 16.97 inches). The smaller magic square fits comfortably into a pocket of most tool belts.
The larger model is especially handy when working on large dimension lumber, two-by-eights and up. Its size makes it handy for laying out framing, when transferring measurements from one wide piece of lumber to another.
Another application for the larger square is in laying out rafters and stairways. An attachment called a layout bar is bolted to the underside of the square, and can be fixed at certain angles (or pitches) for speedy marking of plumb lines or bird’s-mouths on rafters or riser and tread cuts on stairs. This handy extra turns the speed square into a sort of bevel gauge, with one angle preset so that it can be quickly and accurately marked and replicated.
Like the rafter square, a speed square purchased new will come with an instruction booklet that contains rafter tables and explains a variety of techniques for which the tool can be used.