1. I hear people talk about using an M2, M3, or M22s as a center channel speaker, using them vertically and "inverted." I can figure out the vertical, but what is inverted?

2. For those that have used a vertical center channel speaker, how are the results? I am tempted to use an M22 as a center.

Inverted=flipped over, so the tweeter's at the bottom, woofer at the top.

Thanks. How would inverting improve the sound?

It doesn't, necessarily. What it does do is move the bigger magnet away from the top of the TV.

Yes TNT, in my pre-Axiom setup I use a small vertical speaker(most similar to an M2) identical to the mains for the center. When I have the center turned on(rather than just using the mains to create a "phantom" center)it works quite well and in my view has smoother horizontal dispersion than a horizontally configured center(haven't heard VP100 or VP150).

Bingo. Inverting the speaker only makes sense if you have the center placed on top of your TV. In my case it also helped to put the tweeter closer to ear level but that depends on how high your TV is mounted.