A Speed Square (rafter square, rafter angle square, triangle square) is a triangular carpenters' marking out tool manufactured and sold by Swanson Tool Co., Inc. The Speed Square combines common functions of the combination square, try square, and framing square into one. Carpenters use it to make basic measurements and mark lines on dimensional lumber, and as a saw guide for short 45 and 90 degree cuts.
Albert J. Swanson invented the A Speed Square in 1925 as a carpenter's layout tool. He later founded Swanson Tool Company, Inc. to produce it. Today, Swanson Tool Co. still manufactures and distributes the Speed Square and other carpentry tools from its headquarters in Frankfort, Illinois. SPEED and SPEED SQUARE are also registered trademarks of Swanson Tool Co., Inc. 
Common lines made using a Speed Square include perpendicular cut marks, and angles for roofs, stairways, and decks. Embedded degree gradations on the tool eliminate complex trigonometry, making for speedy lines. Swanson makes variants of the tool in aluminum, steel, and composites such as HDPE—and in two basic sizes, the original 7 inch and a 12 inch model for larger tasks.
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 indicates the angle in degrees from 0° to 90°.
- Common indicates the rise in inches over a 12 inch run for common rafters from 1 inch to 30 inch.
- Hip/Val indicates the rise in inches over a 17 inch run for hip or valley rafters from 1 inch to 30 inch.
Some models have divots for fitting a writing utensil to mark lumber with. Genuine Swanson Speed® Squares also have a diamond shape cutout on the ruler side at 3½ inches.
Swanson Tool Co., Inc. describes the tool as a "Try Square, Miter Square, Protractor, Line Scriber, & Saw Guide" in one. The Swanson Speed Square comes with a pocket sized blue reference book that describes the tool's functions and contains charts for rafter lengths and widths from 3 to 40 feet. 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.
This tool uses a 0° reference. This means when a board is squared off the tool reads 0°.The angle derived is actually a complementary angle. For example a 22.5° angle is actually 67.5°. The sum of the angles equals 90 degrees(22.5° +67.5°= 90°). It is obvious from a visual check that where the instruments displays 22.5° is not 22.5°. Many of the new slide miters and miter boxes display both angles. Some of the new calculators have a 0° and a 90° references to do angular calculations. This can create much confusion if the user does not understand this angular calibration.