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ADDENDUM (a) is the height by which a tooth projects beyond the pitch circle or pitch line. BASE DIAMETER (Db) is the diameter of the base cylinder from which the involute portion of a tooth profile is generated. BACKLASH (B) is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth on the pitch circles. As actually indicated by measuring devices, backlash may be determined variously in the transverse, normal, or axial-planes, and either in the direction of the pitch circles or on the line of action. Such measurements should be corrected to corresponding values on transverse pitch circles for general comparisons.