Scott had a number of X-former pix in the Tech ref area @ one time, I found this,,
https://www.electrical-contractor.net/ubb/Forum15/HTML/000044.html

The theory of a 'center tap' is viewable here. Myself, i think it helps to think of electricity simplistically as a circle
( all hardcore theory aside) [Linked Image]

It's just that one point in that circle should be at 0 volts....