What is transformer polarity?

'Polarity' describes the sense (direction) in which the voltage (not current!) induced into secondary winding is acting, relative to the sense in which the voltage applied to the primary winding is acting.

In North America, a transformer's high-voltage winding terminals are identified by the letter H, and the low-voltage winding terminals by the letter X. In the case of a two-winding transformer, the pair of high-voltage terminals are marked H1 - H2, and the pair of low-voltage terminals are marked X1 - X2.

When the potential of HV terminal H1 'goes positive' (i.e. during the first half-cycle of AC), if LV terminal X1 also goes positive at the same time, then the transformer is an 'additive polarity' transformer. On the other hand, if terminal X2 goes positive at the same time as H1, then the transformer is a 'subtractive polarity' transformer.

Knowing the polarity of a transformer is very important if you intend to operate Transformers in parallel with each other (there are other factors, too, of course), in the same way as it is important to know the polarity of a battery before you can connect it in parallel with another battery.