formula, and the fact that a scalar triple product is zero if it contains twice the same vector:
:

Given the definition of the cross product, this means that is the angle in the clockwise direction of the fourth atom compared to the first atom, while looking down the axis from the second atom to the third. Special cases (one may say the usual cases) are , and , which are called the trans, gauche⁺, and gauche^- conformations.