O-rings are the most commonly used seals in fluid-power systems. They must deform to function properly. As the name implies, the O-rings have a doughnut shape. (Torus is the geometric term that describes the O-ring.) They are installed in cavities known as grooves and then compressed. The resulting zero clearance within the groove provides the seal that blocks the flow of liquids and gases. This simple arrangement serves many fluid-power systems very well, but success still requires careful design, selection, and installation procedures.