A Corner cube prism (or retroreflector) is geometrically a cube with a corner cut off, leaving it with three orthogonal faces and a hypotenuse. While a mirror reflects light back to its source only when the incident angle is normal, this prism reflects back onto itself at even large angles of incidence. The reflecting faces are coated to allow almost total reflectance even when the incident angle is large and exceeds the angle for total internal reflection, a principle that these prisms operate on. This makes them very valuable for precision alignment.