Utility power in North America is distributed via single-phase transformers with their primaries being fed 12.47 kV using 2 lines of a 3-phase system or 7.2 kV using a line and neutral.

The distribution transformers, which are pole-mounted, are also known as a 'pole pigs'. 

[![enter image description here][1]][1]

The single secondary winding L1 - L2, with an earthed centre tap designated as the neutral, provides a single-phase 240 V supply between L1 and L2 and two single-phase 120V supplies, which are out-of phase by 180°, between L1 & N and L2 & N. 

Hence it is referred to as a 'single-phase, 3-wire' system or 'split-phase' system.

  [1]: https://i.sstatic.net/pZKEw.png