 Inside diameter is the diameter of the addendum circle of an internal gear, this is also known as minor diameter.Expressed as θ, the involute polar angle is the angle between a radius vector to a point, "P", on an involute curve and a radial line to the intersection, "A", of the curve with the base circle.Expressed as ε, the involute roll angle is the angle whose arc on the base circle of radius unity equals the tangent of the pressure angle at a selected point on the involute.