In my textbook it states that:
In 3 dimensions however there is a fourth possibility. For example, consider two forces whose lines of action are skew (non-intersecting, non-parallel). Such a pair of forces can not be equivalent to a single force, couple or be in equilibrium, but are equivalent to a force and a couple whose plane does not include the force.
I understand the impossibility of equilibrium and it simplifying to a couple, but why can't it be equivalent to a single force acting through a specific point?
A diagram to illustrate would be greatly appreciated.