A support surface's ability to envelop describes its ability to deform around irregularities on the surface without causing a substantial increase in pressure. Examples of irregularities are creases in clothing, bedding or seat covers, and protrusions of bony prominences. A fluid support medium would envelop perfectly. However, surface tension plays an important role in envelopment. A fluid-filled support surface such as a waterbed would not envelop as well as water alone. The membrane containing the water has surface tension. The surface tension has a hammocking effect on irregularities of the interface. Poorly enveloping support surfaces may cause locally high peak pressures that could increase the risk of tissue break down.