We often use incorrect terminology. Take for example the terms "phase" and "polarity" when we talk of flipping the plus and minus of both channels at the same time. What we mean is polarity, but we often use the term "phase". Gary Galo, a reader of these posts and an engineer, sent me an excellent description and picture of the difference between the two. Note in the diagram that A and B in phase differences are moved in time (left to right on the picture), but have the same direction (up and down in the picture). And Polarity Difference changes direction (up and down), but happen at the same time.

We all use the terms "phase" and "polarity" interchangeably, even though we shouldn't. A phase difference is a difference in time, whereas a polarity difference is an inversion. When we reverse the leads on one of our loudspeakers, we often say that the speakers are "out of phase" even though they are really of opposite polarity. The difference between the two is best illustrated using an asymmetrical waveform. I've attached such an illustration - one I've used in several articles and conference papers.