A stroboscope is an instrument used to make a cyclically moving object appear to be slow moving or stationary. The principle is used for the study of rotating, reciprocating, oscillating, or vibrating objects. Machine parts and vibrating strings are common examples. In its simplest form, a rotating disc with evenly spaced holes is placed in the line of sight between the observer and the moving object. The rotational speed of the disc is adjusted so that it becomes synchronized with the movement of the observed system, which seems to slow and stop. The illusion is caused by temporal aliasing, commonly known as the stroboscopic effect.