V-ribbed belts resemble flat belts, but have a series of V-shaped ribs on the drive surface which run the full length of the belt. Unlike flat belts, V-ribbed belts frictionally engage the lateral wedge surfaces instead of the outer pulley diameter. When selecting V-ribbed belts, cross-section specifications are important to consider. English V-ribbed belts are measured in inches and specified by width and thickness. Designations include A or 4L (1/2" x 5/16"), B or 5L (21/32" x 3/8"), C (7/8" x 17/32") D (1-1/2" x 3/4") and E (1-1/2" x 7/8"). English V-ribbed belts that are designated as O, 3L or Z are 3/8" wide. Metric V-ribbed belts are measured in millimeters (mm) and specified by width and thickness. Choices include SPA (13 mm x 8 mm), SPB (17 mm x 11 mm), SPC (22 mm x 18 mm), and SPZ (10 mm x 6mm). Metric V-ribbed belts with widths of 5, 6, 8, 10, 13, 17, 20, 22, 25, 32, or 40 mm are also available.