A speed square or rafter angle square is a triangular-shaped tool which combines some of the most common functions of the combination square, try square, and framing square into one. It is used to make basic measurements and mark lines on dimensional lumber, and as a saw guide for making short 45 and 90 degree cuts.
The tool was invented in 1925 by Albert J. Swanson and readily earned its name in use. While Speed Square is a registered trademark of the Swanson Tool Co., Inc., it has become a genericized trademark for similar products.
The tool is a right triangle with a ruler on one equal side and a fence on the other. It is marked with the word Pivot at the right angle point and displays Degrees on its hypotenuse, Common and Hip/Val markings on its midsection.
- Degree indicate the angle in degrees from 0° to 90°.
- Common indicate the rise in inches over a 12 inch run for common rafters from 1 in. to 30 in.
- Hip/Val indicate the rise in inches over a 12 inch run for Hip or Valley rafters from 1 in. to 30 in.
Some models have divots for fitting a writing utensil to mark lumber with. Genuine Swanson Speed Squares will also have a diamond shape cutout on the ruler side at 3½ in.
Swanson Co. describes the tool as a "Try Square, Miter Square, Protractor, Line Scriber, & Saw Guide" in one. Swanson Speed Squares come with a pocket sized blue reference book describing the tool's functions and containing charts listing rafter lengths for building widths from 3 to 40 ft.
Among its basic uses are marking Common, Hip, Valley and Hip, or Valley Jack rafters, laying out stair stringers, determining and marking angles, and making square cuts on boards.