the.com/speed square

means A flat triangular tool carpenters use to mark and check 90° and 45° angles for cutting lumber and framing rafters.

from Also called a rafter square or triangle square, it was popularized in the early 20th century — commonly credited to a patent by Swanson Tool Co. in the 1920s-30s — as a faster alternative to the bulkier try square and framing square, combining angle-marking, common-rafter, and protractor scales into one pocket-sized triangle; the name simply marks what it's for: speed.