Consider two vectors x = {{a},{b}} and y = {{c},{d}} which are both of the same length. Then there is a rotation matrix R(theta) which takes x to y, namely R(theta)x = y. We note that 0 =< theta < 2pi. Use our knowledge of rotation matrices to establish a simple condition on a; b; c; d so that the angle 'theta' satisfies 0 < theta < pi