meaning that at the stationary point, the variation of the action A with some fixed constraints C is zero. For action principles, the stationary point may be a minimum or a saddle point, but not a maximum. Elliptical planetary orbits provide a simple example of two paths with equal action – one in each direction around the orbit; neither can be the minimum or "least action". The path variation implied by δ δ is not the same as a differential like d t . The action integral depends on the coordinates of the objects, and these coordinates depend upon the path taken. Thus the action integral is a functional, a function of a function.