- A rule that uses the shape the right hand to established the standard orientation of vector quantities normal to a plane, especially when calculating a vector product or the helicity of particle spin. In the case of the vector product C = A X B, the direction of C is obtained by pointing the right hand with fingers straight in the direction of A, and then bending the fingers in the direction of B; the extended thumb now roughly points in the direction of C. For spin, the fingers of the right hand should curl in the direction of motion, and the thumb shows the direction of the spin vector.