The diameter of an imaginary circle on which the gear tooth is designed is the ____.

A) diametral pitch
B) pitch diameter
C) root diameter
D) circular pitch

The pitch diameter is the imaginary circle on which the gear tooth is designed. It is the diameter that is used in calculating the gear ratio and in determining the tooth size and spacing. The diametral pitch is the number of teeth per inch of pitch diameter. The root diameter is the smallest diameter of the gear at the bottom of the tooth spaces. The circular pitch is the distance between corresponding points on adjacent teeth measured along the pitch circle.