A machinist square or engineer's square is the metalworkers' equivalent of a try square

Try square

A try-square is a woodworking or a metal working tool used for marking and measuring a piece of wood. The square refers to the tool's primary use of measuring the accuracy of a right angle ; to try a surface is to check its straightness or correspondence to an adjoining surface...

. It consists of a steel blade inserted and either welded or pinned

Rivet

A rivet is a permanent mechanical fastener. Before being installed a rivet consists of a smooth cylindrical shaft with a head on one end. The end opposite the head is called the buck-tail. On installation the rivet is placed in a punched or pre-drilled hole, and the tail is upset, or bucked A rivet...

 into a heavier body at an angle of 90°. In the accompanying image, there is evidence of pinning at the intersection of the blade and body: the heads of three pins are visible as dark circles. A small notch has been added to the inside corner of the square to prevent small particles from accumulating and affecting the square's reading. This notch is typical of machinist squares and is occasionally found on carpentry squares as well.

Use

In use, the body is aligned against the one edge of the object and the blade is presented to the end or body of the object. If the end is being checked, then a strong light source behind the square will show any mismatch between the blade of the square and the end of the object. The purpose of this action is to either check for squareness or to mark out

Marking out

Marking out or layout is the process of transferring a design or pattern to a workpiece, as the first step in the manufacturing process...

the body of the workpiece.

Accuracy

Machinist squares should have a linear error of no greater than 0.0002 in/in.

Squares must be occasionally checked for accuracy. The four disk method is one way to verify overall squareness. However, it cannot detect bent blades.