Bill, Yes, the source is 120/208 3Ph Wye, but often only 2 phase and Neutral is brought into the residential installation.
I submit that you mean single-phase or "split phase", centre-tap to Neutral, also Earthed at the entrance (only). That's North American & Japanese practice, mostly. Europe
et al work directly off two of the 3-Phase legs @ 230-250 VAC for residences, but do not ordinarily provide 120 VAC service at a wall outlet.
TWO phase still exists, BTW, but only "barely".
With need of four, not three power conductors as 3-Phase uses, the Copper cost of two-phase was never as popular with investors and their bean-counters when 3-Phase was both smoother and cheaper to distribute.
"Really Cheap" is single-phase,
one wire, with "phantom" return through the Earth, same as a simplex-phantom [1] telegraph cable.
Cost issue, again - Andes mountains, mostly.
"Back in the day" it was seen as better than no electricity at all - and "oh, BTW" could also provide that telegraph, later voice telephone service, later-yet, data - all over that same solitary wire - to a village, police post, border outpost or such.
An IC engine PLUS its fuel and lube ever-after, far the more enduring challenge - wudda needed packing up the mountain on animal back, be it four-legged "animal" or only TWO!
[1] Probably MOST of the internet gets the descriptions wrong in co-joining the terms that LONG predate radio.
"Simplex" is a "one-way" (at time..) circuit. Radio, clear into War Two, Citizens band, aircraft, walkie-talkie.. "over.." ... plus most early submarine telegraph cables were so configured, then operated send-only one way part of the day, laboriously switched-over on rigorous schedule, operated the other direction the other half.
"Laborious" as some of them needed 8,000 Volts in at the sending end to deliver a gnat's-fart worth of signal to the other end, and at as little as FOUR words per minute, even so, inductances as they were, 1898, completion of the world's first more-or-less "global internet".
Faster than steamship? Not always. Typical 2-week wait in the queue to get a commercial message to the telegrapher's key. Longer-yet if the priority-holder - the British Government - had a war on or something similar that took up all the bandwidth.
"Phantom" is just what it sounds like. No visible conductor. Full-duplex phantom has existed, so it is not limited to "simplex."
The capacitance, not resistance, of the Earth's skin is used as a "sink" to
simulate a conductor, given it takes a Hell of a lot longer to "charge" such a lossy capacitor it never much notices that we puny humans are even making the attempt. Earth is a conductor with (usually) high resistance as well, but that is more hazard than help.