A pulse is a type of wave that is limited in its extent. A physical example which illustrates pulses is a long taut string. When plucked, a disturbance, consisting of particles moving perpendicularly to the string, travels along the string. This disturbance is localized, which means that particles in the string only move near the center of the wave. Pulses are often constructed so that they are very short; this means that the pulse is highly localized in a very narrow spatial region, so the entire pulse will pass by a given point on the string very quickly. Pulses may be measured in seismic waves from an earthquake, or a light pulse may be used to cause vibrations in a solid crystal.