The cross section of the O-ring can be measured in both the axial and radial direction. A face seal uses the axial cross section, or face of the O-ring to seal. When using an O-ring as a face seal, a groove is cut in a flat surface, such as a plate. The O-ring is placed in the groove and a second flat surface compresses the O-ring. An O-ring in a face seal design is static, meaning that that once installed, the O-ring does not move. While the application pressure may cause the O-ring to shift slightly, this is still considered a static seal. Face seals can seal internal pressure, external pressure, or both.