Bend classifications are divided based on their curvature radius, into long-radius and short-radius bends. Long-radius bends refer to those with a curvature radius equal to 1.5 times the pipe's outside diameter, i.e., R = 1.5D. Short-radius bends refer to those with a curvature radius equal to the pipe's outside diameter, i.e., R = D. In this formula, D is the bend diameter, and R is the curvature radius. When classified by pressure ratings, there are approximately seventeen types, which are the same as in U.S. pipe standards, including: Sch5s, Sch10s, Sch10, Sch20, Sch30, Sch40s, STD, Sch40, Sch60, Sch80s, XS; Sch80, Sch100, Sch120, Sch140, Sch160, XXS, with STD and XS being the most commonly used. By bend angle, there are 45° bends, 90° bends, and 180° bends. This results in a wide variety of bend types. When placing an order, the order form often uses the following notation: for example, "LR STD 90° 8" indicates a long-radius bend with a pressure rating of STD, an 8" bend at 90°; or, "SR XS 45° 4" indicates a short-radius bend with a pressure rating of XS, a 4" bend at 45°. The above is a general classification of bends.