Also known as T-rings, T-seals (see Figure 143) are used with pressure-energized back-up rings (typically made of nylon or PTFE) on both sides, making them ideal for dynamic sealing. Harder (higher shear modulus) back-ups can be used than with O-rings, and the forcing of these back-ups into the clearance gap makes T-seals very extrusion-resistant. The T-shaped cross-section adds stability, so T-seals are more resistant to spiral failure than O-rings. Chief downsides: T-seals require installation of three parts (the seal and two backups), and they cost much more than O-rings.