This British Standard specifies requirements for engineers’ try squares
(excluding adjustable squares), cylindrical squares and block squares of
solid or open form.
Section 1 specifies general requirements applicable to all types of
square.
Section 2 specifies requirements for engineers’ try squares in
three grades of accuracy, AA, A and B, with inner lengths of blade
from 50 mm up to and including 1 000 mm.
Section 3 specifies requirements for cylindrical squares in accuracy
grade AA in sizes from 75 mm up to and including 750 mm.
Section 4 specifies requirements for solid form block squares in
accuracy grades AA and A in sizes from 50 mm × 40 mm up to and
including 1 000 mm × 1 000 mm.
Section 5 specifies requirements for open form block squares in
accuracy grades A and B in sizes from 150 mm × 100 mm up to and
including 600 mm × 400 mm.
NOTE While the standard provides for squares in various grades of
accuracy, it should be noted that their accuracy in use is dependent upon
the flatness of the surfaces on which they are used.
BS 939:2007 Referenced Document
BS 1134-1 Assessment of surface texture. Methods and instrumentation