Metric O-Rings are employed in specialized applications and, like the name suggests, are standardized by metric dimensions. With a wide variety of sizes available, Metric O-Rings fill in the gaps left by the standardized AS568 o-ring chart. Metric O-Ring sizing is determined by the o-ring's inside diameter and cross section width. PTFE is also know as TFE and Polytetrafluoroethylene. For more information, please see: Comparison Properties Chart, PTFE Encapsulated O-rings, Material Shelf Life, and O-ring Groove Design Chart.