They are in fact exactly IN phase with each other, meaning that there is only ONE phase. The two legs of a single phase 240vac circuit, like the one found in most houses, are NOT out of phase with each other. The line voltage here has been 123V since 1960 or so. Power companies like to keep the voltage as high as possible to get the maximum use of their distribution network. The present US standard is 120V +/-5% or +/-6V, or 114V to 126V. By 1926 110V lamps accounted for only 12% of demand (35% were 120V). Over time, higher voltages predominated, and by 1919 115V overtook 110V in popularity. Various cities were encouraged to standardize on different voltages, from 110V to 125V, to create a market for all lamps produced. That's where all the multiples of 11 came from (110, 220, 440, 550, 2200).Д®arly carbon-filament lamps could not be produced uniformly, and the lamps were selected at manufacture to suit a particular voltage. Historically Thomas Edison selected 100V for his lamps, and allowed 10% drop in the supply lines, so the generators produced 110V. 115V and 230V were standardized in 1928 and before that, there was a mix of 'standard' voltages from 110V to 125V. The present standards are 120V and 240V and have been since the 1950s. It's difficult to explain in words without reference to diagrams, and math or at least vectors.ДЄt the risk of confusing the issue, 'one-ten' and 'two-twenty' volts are figures of speech and have not been used for eighty or so years. True two-phase is an antiquated system (Tesla's original multiphase motors were two phase) that has largely been supplanted by three-phase. 120V/240V is 3-wire single phase, not two phase. The entire system is synchronized.Д¬lick to expand.It's because the term 'phase' is being used to describe two different things. Basically, it does not vary and if it does, it happens simultaneously all through North America. The frequency (60 Hz in North America) will not change and if it ever does, there are serious, serious problems going on in at the generation facility -to the extent you will probably see a bright flash coming from the facility. 110 and 120 are the same really it's just that the power company is allowed to vary the voltage under different circumstances and it could be somewhere in that range. Without a significant description of phasor mathematics and something called 'complex numbers', it would be very hard for me to describe in other terms why the phase change occurs but, suffice it to say, it does indeed occur.Д«TW, the terms 110, 120, 220 and 240 are a little confusing. If you measure between the neutral wire and any hot wire, the voltage is 110 but, one with respect to the other is 180o out of phase. That wire is the neutral that is brought into your house. In the middle of that winding, they put a 'center-tap' which is just a wire that connect to the center of the winding. If you measure the voltage between those two hot wires, it will read 220 volts. The two wires from that winding are the two hot wires that enter your house. They put a winding next to the core and it induces 220 volts into the winding. Somewhere in the neighborhood, there is a transformer with high voltage coming into it. The 220 power that we are familiar with is what comes into our home service panels and it consists of one neutral and two 'hot wires'. Two phase is not the correct terminology and sometimes, the terms 'split phase' or 'split leg' is used. The following applies to North American electrical systems.

European 220v vs us 220v