A try square is a tool used in woodworks and engineering or metal works. It is used to mark and measure a piece of wood or metal.
A try square is composed of two parts, the stock and the blade. This tool is used to measure the accuracy of a 90 degrees angle or a right angle.
The try square is called such because the users of this tool will "try" to check if a surface is straight and if it corresponds to its adjoining surface or to check how "square" it is. Square refers to the right angle of the wood or steel.