Machinist square

Under both these standards there are basic dimensions set out for blade/body length ratio, and cross sections of both, as well as different types of squares such as those with a solid body and an inserted blade, or those with a single piece blade and body being precision ground with another plate attached to the base of this to provide a square 'butt' to refer to.

The simplest method of checking a low grade square for accuracy is to draw a line across a board with a flat face and straight edge (illustrated).

By nature of their construction a cylindrical master square cannot help but sit at a perfect 90-degree angle to the flat surface.

A square to be tested can be presented to the master and feeler gauges used with a source of light behind the gap to get a reading of deviation.

If done cleanly and correctly, this method can find errors down to 0,01 mm, which is the limit of practical accuracy since thermal expansion from contact with the fingers will distort the blade by approximately that much.