The two forces at B will cancel, and we are left with a couple of moment P.AC in the plane AC. If we draw three **vectors** to represent these three couples, they will be perpendicular and proportional to the respective sides of the triangle ABC; hence the third vector is the geometric sum of the other two.

Newtons Second Law asserts that change of momentum is equal to the impulse; this is a statement as to equality of **vectors** and so implies identity of direction as well as of magnitude.

These may be compared and contrasted with such quaternion formulae as S(VabVcd) =SadSbc-SacSbd dSabc = aSbcd - bScda+cSadb where a, b, c, d denote arbitrary **vectors**.

In particular, we infer that couples of the same moment in parallel planes are equivalent; and that couples in any two planes may be compounded by geometrical addition of the corresponding **vectors**.

A cyclic convolution requires that the lengths of the input **vectors** are identical.