Other factors tend to compress waves. When ocean waves approach the shore, for example, the shallow bottom may slow down the waves and cause them to pile up, much as a loose pack of racehorses bunches up when the front-runners suddenly encounter rough terrain. Under the right conditions, the effects of dispersion and compression balance out, producing an unusual wave called a solitary wave or soliton. A soliton has a single humplike crest but no trough. Solitons do not readily disperse, but instead keep their size and shape much longer than ordinary waves do.
Solitons often form in shallow water channels.
Most waves, such as water waves, travel from one place to another. These waves are called progressive waves. Under certain conditions, however, waves can be trapped in a medium. For example, if a rope is held at both ends and plucked, the energy in the waves cannot leave the rope at either end. This trapping of waves results in patterns of vibration called standing waves.