Given two planes, let n1 and n2 be vectors normal to the two planes. Then a method of obtaining the angle between the planes is to take the angle between n1 and n2 (see angle (between vectors)), with the directions of n1 and n2 chosen so that the angle θ satisfies 0 ≤ θ ≤ π/2 (θ in radians), or 0 ≤ θ ≤ 90 (θ in degrees).