A try square is a wood worker's tool used to either test to see if an edge is squared or to mark a piece of wood for cutting. The wood is marked all the way around in order to cut joints or simply to cut the end of a piece.
The word "square" in the name of the tool alludes to use of the tool to measure the accuracy of 90-degree angles. The try square and piece of wood are held up to the light and then the tool is moved along the edge of the wood. If light is seen showing between the tool and the wood, the edge is not square. The word "try" alludes to the use of the tool for trying the edges of cut wood to make sure it is square. Try squares are sometimes incorrectly referred to as "tri" squares, but the root of the word is in "trying."
Try squares are, traditionally, made of a wooden handle, or a stock, with a broad steel blade riveted to it. The inside of the stock is usually fixed with a brass strip to prevent wear on the try square. Sometimes, the steel blade is marked with measurements. Less traditional try squares may be made entirely of metal.