English: Diagram of the "clock number" of a 3-phase transformer, indicating, as multiples of 30°, the phase shift between the primary and secondary voltage of a three-phase transformer. The diagram shows a representation of a transformer with clock number of 5. The primary vector V of the primary coil triangle points to 12 and vector v of the secondary star to 5 hours, so that in this case a phase shift of 5×30° ${\displaystyle =}$150° is present.