The typical structure of the eukaryotic flagellum consists of a central pair of singlet microtubules surrounded by nine doublet microtubules, called the axoneme. Much has been discovered regarding the mechanism by which axonemes produce motion: ATP is used by dynein arms found on the A tubules of the doublet microtubules to produce shear force against the B tubules. These shear forces are then converted to bending. However, if all the dynein arms along the length of the axoneme and on all doublets attempted to produce shear simultaneously, no effective movement would result. Thus, regulation of active shear force is required. Evidence suggests that the central pair–radial spoke complex is involved in this regulation. The first evidence came from an electron micrograph study in which the central pair microtubules of Paramecium, “instantaneously fixed” and serially sectioned, appeared to be oriented in systematically changing angles. This was interpreted as rotation of the central pair with respect to the nine outer doublets per beat cycle (Omoto and Kung, 1979 , 1980 ). It was suggested that the central pair may act as a “distributor” to regulate the activity of dyneins.