A pair of equal, parallel and opposite forces which are not in the same straight line. A couple provides a pure torque or turning effect. The value of the couple is given by the joint turning effects of the forces about the mid-point between them, which is the same as the product of their distance apart and one of the forces. Sometimes it can be convenient to introduce the idea of a couple to an arrangement of forces to make clearer what is happening. As a simple example, consider the following:
Suppose a force P acts down on the end of a cantilever of length L. Apply equal and opposite vertical forces P1 upwards and P2 downwards at the support. (On their own these would have no effect.) Then P and P1 together form a couple (P x L) tending to turn the beam , and the remaining force P2 tends to push the beam downwards. Thus, the load P has both rotating and translating effects on the beam.