Transformer Polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage is from X1 to X2 the secondary induced voltage direction will be from H1 to H2.In practice, polarity refers to the way the leads are brought out of the transformer.  An understanding of polarity is essential to correctly construct a three-phase transformer from single-phase transformers.