the channels they are a-changin

When recording, you have to keep in mind the different ways people listen.    Whatever you do has to sound good in cars, or on earbuds, or on a little speaker, or a great stereo, or an iPad, or a zillion other things.    Most of the time, a mix that sounds great on a low-fi system will sound great on an HD system.    But that's not always true going from stereo to mono, where you get phasing. 

Picture what happens when you put one window screen in front of another, or one comb in front of another.    Weird patterns emerge as the grids overlap and interlock. 

So, think about it: in stereo, there are two channels, left-ear and right-ear.    When you overlay them, there are some parts that may react like those screen doors.

I was talking with an engineer about this —– a dude who was placing mics for a small ensemble in an ORTF array (good stereo but weird and phasey if you're listening in mono) —– and he said, "ahh well everyone listens in stereo these days, from headphones to earbuds to car stereos."   That's out-of-date!   Absolutely true 5 years ago, but these days lots of people (and I mean LOTS according to the stats) listen on home devices like Alexa, which is gosh-durn mono.    So you gotta be able to provide rich stereo sound, but, crucially, a sound that sums to a rich, non-weird mono.