A tap here can also mean a pair of lines from one winding of a transformer. So, typically, our reduction transformers have 2 taps each one at 120 v and 60 cycles, but being of opposite "polarity" in terms of position on the sine wave so that one is high when the other is low giving a difference of 240 v across the span of the tap. It is like an "E" where the top and bottom legs are powered and the center is ground. The voltage across the top and bottom is 240V and across either end and the center ground is 120.