An optical square is a device which displays a fixed angle, usually 90 degrees between two objects (or 2 times 90 degrees between three objects). The plumb level is a special form as it gives the angle relative to the vertical. The most common of the squares is the hexagon or pentaprism giving one or two right angles for staking out. Also included are the equerre and pantometer that both uses a series of slits setting out 45 and 90 degree angles. Finally the roof and zenith plummets can be considered optical squares as they measure exactly vertical (and nothing else).
Most optical squares in my collection are pentaprism instrumetns from Wild Heerbrugg.