putting speaker vertical or horizontal?

Horizontally-arrayed drivers which are farther from each other than 1/2 wavelength, based on frequencies which both play (for example, at crossover), will result in a pattern of cancellations and reinforcements in the horizontal plane. This results in varying frequency response as one moves even slightly off dead-center. Besides, both ears cannot be dead center at the same time, anyway.

Kal

Would they image? Possibly or we would not have stereo, would we? OTOH, that is with slightly different signals. If you compare a mono signal over the pair with a mono signal over a single centered driver, you can hear the difference in terms of image stability with respect to head movement.

As for the ubiquity of MTM centers, that is a model supported by its visual, rather than its sonic, impressions for accomodation to those with the common space constraints. The only good centers have vertically stacked tweeters and mid-ranges. A standard speaker configuration is even better.

Kal