In the U.S.: power from the utility comes in 3 “hot” power lines plus a neutral return. (Actually this isn’t quite the way it works, but I’m simplifying.) Each line delivers 120 volts relative to the neutral, but 208 volts relative to the other lines. Some appliances require 208 volts (e.g. electric dryers and ranges), so the utility delivers 2 of the 3 hot lines plus a neutral to each house. The 208 volt appliances get 2 hot lines and a special plug and outlet configuration so you don’t accidentally plug things into the wrong outlet. Everything else in your house uses 120 volts, so all of the other outlets (i.e. almost all of them) have a single hot line and a neutral going to them. (The hot line goes on the smaller prong of a standard U.S. NEMA 15 plug/outlet configuration.) But two hot lines are going to your house and many people don’t even have any 208 volt appliances (e.g. if they use gas appliances) so to balance power consumption on each line, the house is wired up with roughly half of the outlets being fed from one line and half from the other. (Usually every other breaker in your breaker box is on a different line.) Powerline across the lines does supposedly work, but it relies on antenna pickup between the phases at the transformer, which seems sketchy to me.