Abstract: A precision optical square instrument for accurately determining lines and planes respectively perpendicular to reference lines and planes comprising, a substantially hollow housing containing first and second reflectors arranged at different vertical positions for viewing first and second marker positions outside the housing which are substantially at right angles to each other. The images of the two marker positions are projected onto a third reflector which is viewed by a user, such that the user can determine when the two marker positions are located perpendicularly, by the alignment of the images viewed in the third reflector.