Fig. 2.

Projection of the angle in space.

∠AOB is a plane angle in any space, where A and B are the intersection points of both sides of the angle and surface H, respectively. ∠AoB is its projection on surface H. The three points, namely A, o, and O, form plane N. The three points, namely B, o, O, form plane M. Both plane N and plane M are perpendicular to plane H. ∠AOB = θ₁, ∠AoB = θ, OA = R₀, OB = r₀, oA = R, oB = r, Oo = h.