A ring with its perimeter graduated in angular measure and placed perpendicular to a vertical axis of rotation of an instrument, with the axis passing through its center. Horizontal circles of theodolites were formerly made of metal and were read through two microscopes placed 180^o apart. In modern theodolites, the horizontal circle is made of glass and is read through a single microscope which, by a prismatic train, lets the observer see the two opposite sides of the circle simultaneously.