a very simple geodetic instrument that is used to lay off angles that are multiples of 90° or of 45°.

In a prism square (see Figure 1), the direction of a ray originating from the flag on the right is changed by 90° after refraction and internal reflection. To lay off the angle by which the direction of the ray is changed, a second flag is set up in such a way that, when it is seen over the instrument, it is exactly above the image of the first flag in the instrument.

Figure 1. A prism square

Optical squares are used to solve geometric problems on a terrain and to carry out plane surveys.