An Optical Square is a hand-held instrument that allows the user to see at right angles from a line of sight. It is useful for setting out right angles (eg corners of buildings), locating offsets from a traverse line and for measuring cross-sections along an alignment. It relies on the principles of reflection and refraction, where the light coming into the instrument is reflected off one surface, onto a second, off the second and out the instrument at right angles to the direction it came in. At what angle(s) do the reflecting surfaces in the Optical Square need to be set in order to get 90 degree reflection through the instrument?