Timing pulley flanges are used to maintain belt contact with a timing pulley in power transmission applications. Typically, stock pulleys include options for flanges. Separate flanges are available for custom pulleys made from pulley stock. Timing pulley flanges are manufactured to fit timing pulleys of the same pitch and size. The dimensions of a pitch, including the mating flange, are specified by the number of grooves. Pulley pitch is the distance from the midpoint of one tooth to the midpoint of an adjacent tooth. Timing pulley flanges are available in both English and metric units. English pitch is specified in inches (in.) or fractions of an inch. Lettered designations include L, H, and XL. Metric pitch is specified in millimeters (mm). Letter designations include GT and HTD.