 For external gears, the addendum circle lies on the outside cylinder while on internal gears the addendum circle lies on the internal cylinder.Apex to back, in a bevel gear or hypoid gear, is the distance in the direction of the axis from the apex of the pitch cone to a locating surface at the back of the blank.The back angle of a bevel gear is the angle between an element of the back cone and a plane of rotation, and usually is equal to the pitch angle.