A few words on how this device works would probably help. The squareness gage is placed on a surface plate along with the object to be tested. It is then levelled accurately so that it is exactly at right angles to the surface plate, which also has to be levelled, of course. As an alternative, the squareness gage can be set of to be parallel to the vertical face of a known square resting on the surface plate. A dial indicator is attached to the movable arm of the squareness gage and zeroed at the bottom of the face of the object that is being tested. Then the movable arm is raised, and any changes in the reading of the dial indicator are noted. If it is possible to rotate the object under test by 180 degrees about a vertical axis, it is easy to check whether the squareness gage has been set up properly by repeating the dial indicator readings. Then take the difference between the two sets of readings to find out the squareness of the test object's face. A search on "squareness gage" will turn up one or two other manufacturer's similar devices, along with more detailed explanations of their function. Unfortunately, VEEKO's web site for this gage, listed below, is not of much help in this regard.

The squareness gage is based on a granite straightedge enclosed inside its housing, along with a sliding arm that stays in accurate contact with one edge of the straightedge as it is raised and lowered. The feet of the housing are adjustable so as to make the straightedge truly perpendicular to the reference surface provided by a surface plate.