Types of elbows are classified by their radius of curvature, into long-radius elbows and short-radius elbows. Long-radius elbows refer to those with a curvature radius equal to 1.5 times the pipe's outside diameter, i.e., R = 1.5D. Short-radius elbows refer to those with a curvature radius equal to the pipe's outside diameter, i.e., R = D. In the formula, D represents the elbow diameter, and R represents the radius of curvature. 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 commonly used. By angle, there are 45° elbows, 90° elbows, and 180° elbows. This results in a wide variety of elbows. When placing an order, the specification often takes the form: for example, "LR STD 90o 8" indicates a long-radius elbow with a pressure rating of STD, an 8" bend; or "SR XS 45o 4" signifies a short-radius elbow with a pressure rating of XS, a 4" bend. The above is a general classification of elbows.