Let P(x, y, z) be any point in space with reference to a rectangular coordinate system O (XYZ). Let α, β and γ be the angles made by OP with the positive direction of coordinate axes OX, OY, OZ respectively. Then cos α, cos β, cos γ are called the direction cosines.

**Sum of the squares of direction cosines is unity.**

cos^{2}α + cos^{2}β + cos^{2}γ = 1

**Sum of the squares of direction sines is 2.**

sin^{2}α + sin^{2}β + sin^{2}γ = 2