Originally Posted by cguidry21
That's portrait mode vs landscape mode. Video was never intended to be taller than it is wider, unlike still photography.
WMP probably can't deal with the non-standard aspect ratio of portrait mode video. I believe YouTube can deal with portrait mode videos, only thing is you get a tall video with huge black borders.
This issue has really come about since the advent of video capable phones. Because back in the day you wouldn't usually hold a cine-camera or camcorder sideways. Historically TVs and movies have always been landscape mode(except for Baird), unless you turn your TV on its side.