The influence of various design parameters on the cogging torque developed by permanent magnet machines is investigated. It is shown that the slot and pole number combination has a significant effect on the cogging torque, and influences the optimal value of both skew angle and magnet are, as well as determining the optimal number of auxiliary teeth/slots. A simple factor, which is proportional to the slot number and the pole number and inversely proportional to their smallest common multiple, has been introduced to indicate the "goodness" of the slot and pole number combination. In general, the higher the "goodness" factor the larger the cogging torque.