Interaction of one or more pumps (primary element) with one or more control elements (secondary element, in most cases a hydraulic motor). The distance between the two devices is of no significance (open transmission). If pump and motor are close together within a confined space, the term used is integral hydrostatic transmission.
A hydrostatic transmission may be based on the combination variable pumpfixed displacement motor, (primary control), constant pump variable motor (secondary control) or variable pump — variable motor (primary and secondary control).