Observe the following rules when using a combination square:
Grasp the anvil with one hand, so your fingers and thumb are behind the 90-degree reference surface.
The anvil is designed to be grasped with one hand. Its rear portion is S-shaped, so it fits neatly into the palm of your hand and hooks around the base of your index finger. This grip places the index finger and the thumb on opposite sides of the lock knob so that it can be easily manipulated.
The primary function of the combination square is to provide a standard against which to compare the perpendicularity of two surfaces. Its reference surfaces are on the anvil, and are the large wide surfaces that form 90- and 45- degree angles with the rule. Place the anvil, not the rule, against the reference surface of the work piece when checking for perpendicularity.
A common mistake when using the combination square to check for perpendicularity is to use the edge of the rule as a reference surface instead of the large wide face of the anvil. When making the corner of a work piece square, one of the surfaces comprising the corner should be made uniformly flat and designated as a reference. Then make the surface adjacent to it square to the reference by using the combination square as a guide.
The large wide face of the anvil should be placed against the reference surface of the work piece, and the edge of the rule should be used to check the orientation of the adjacent surface. The wide face of the anvil provides a short, stable platform with which to anchor the combination square against the work piece reference, and the rule can be extended to check a great distance along the adjacent edge.