The try-square is a precision instrument which is used to check the squareness of a surface and the flatness of surfaces.

A try- square is a woodworking tool used for marking and measuring a square piece of wood. The square refers to the tool's primary use of measuring the accuracy of a right angle (90 degree angle); to try a surface is to check its straightness or correspondence to an adjoining surface.
Try square is used to check right angle, not to measure.