A carpenter’s square, or squire, is an L-shaped instrument, used for truing right angles; the edges are often marked as rulers, though this is artistic license. It is found in the arms of Atlow, c.1520 [DBA2 401]. A similar charge, the “engineer’s square”, is found in the canting arms (German Winkel) of Wynckler or Winckler, c.1450 [Ingeram 25; also Siebmacher 161].
The square doesn’t seem to have a default orientation; it must be specifically blazoned; but period examples always display the square with one arm vertical and the other horizontal. The illustration shows a carpenter’s square, point to dexter chief.
Thomas Quilliam bears: Azure, a carpenter’s square, point to chief argent.
Maimuna al-Bukhariyya bears: Argent, a carpenter’s square palewise, corner to dexter chief gules.