Given two lines in space, let u1 and u2 be vectors with directions along the lines. Then the angle between the lines, even if they do not meet, is equal to the angle between the vectors u1 and u2 (see angle (between vectors)), with the directions of u1 and u2 chosen so that the angle θ satisfies 0 ≤ θ ≤ π/2 (θ in radians), or 0 ≤ θ ≤ 90 (θ in degrees). If l1, m1, n1 and l2, m2, n2 are direction ratios for directions along the lines, the angle θ between the lines is given by