Every seal, whether static or dynamic, must seal against at least two contacting surfaces. In static applications, both surfaces are non-moving relative to one another. In dynamic applications at least one surface is in motion relative to the other sealing surface(s). For example, in a standard hydraulic cylinder, the rod and piston seals would be classified as dynamic seals, while the seal between the bore and the head gland would be considered a static seal.

In both static and dynamic applications, a certain amount of squeeze or compression is required upon installation to maintain contact with the sealing surfaces and prevent fluid leakage. Dynamic applications in particular involve other variables and require that additional factors be evaluated to ensure proper system performance. These variables are discussed in the following sections.