The paper deals with polyphase commutator machines, treated as a single subject, and shows how all the different types can be classified and analysed within a common theoretical framework.The classification is made into three general groups depending on the construction of the machines, namely the shunt and series motors, the commutator frequency changer and the Scherbius machine. The second group includes the well-known Schrage variable-speed motor, and the third group includes many types of speed-regulating machines and phase advancers. After discussing this classification, the paper describes the practical arrangement of each type of machine and the associated regulating apparatus, and explains the general effect on performance. It is shown that two curves, the speed/torque curve and the current-locus curve, tell all that it is required to know about the performance of a machine.In the theoretical analysis an even greater unity of treatment is possible. The paper shows that the theory of all the important types of polyphase commutator machines can be derived from that of a slip-ring induction motor which has a slip-frequency voltage injected into the secondary winding. The vector diagrams and equations of the principal types are developed by this method; the equations check with those derived by other methods.The chief differences between the three groups are brought out in the characteristics. Families of speed/torque and current-locus curves are shown and compared for some of the principal types.