Progressing cavity pumps are classified as single-rotor, internal-helical-gear pumps within the overall category of positive displacement pumps. The stator always has one more "tooth" or "lobe" than the rotor. Currently, the vast majority (i.e., estimated at 97%) of PC pumps in use downhole are of the single-lobe design and thus are the primary focus of this page. Other variations of these basic configurations include semi-elliptical rotor/stator geometries and uniform-thickness elastomer pumps. The geometric design of a single-lobe PC pump is illustrated in Figure 1.