Avoid misunderstanding the concept of phase by asking yourself, could a waveform exist across a cylinder. There are lots of times when polarity and phase are really interchangeable. But where it matters, it matters because you’re referencing a 360 degree shift. This only makes sense in a periodic waveform where you can illustrate the beginning and the end of that wave as being connected. And to do that, it’s also going to pass through the center line twice.
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