Transversal pulses (above) and longitudinal pulses (below). Above, if we strike one side of the string upward or downward once, a pulse in the form of the crest or the trough is formed and travels along the string. Note that despite pulse propagation, the string itself is not displaced. Below, if we strike one side of the spring to the right or to the left once, a pulse in the form of a spring compression or a spring rarefaction is formed and travels along the spring. Note that despite the pulse propagation, the spring undergoes only a small net movement.