The Rafter angle square is a three-sided, triangular shaped measuring tool, that can be used to draw straight lines on lumber to be cut, or to lay out angles for roofs, stairways, decks and other general construction. It was invented in in 1925 by Albert J. Swanson. He called it the "Speed Square," since it provides a short cut for measurements and calculations of angles.
Generally, the tool comes in two sizes - the compact 7 inch version for portable use and the larger less portable 12 inch version for stationary work. They may be made of aluminum, steel, HDPE, and other composite materials.