Quote Originally Posted by zencentury View Post
Vertical. I just need to drop the wheel back in again, thanks.

What kind of bike would have horizontal dropouts? Seem like that would make it hard to remove the wheel, like the derailleur would be in the way.
Any road bike more than 10-20 (?) years old. Not sure when most manufacturers switched, but most bikes of the 70s and 80 and maybe 90s had horizontal dropouts. They are not completely horizontal, maybe 30 degrees off - the wheel slides forward and a bit down before it comes out. The forward motion clears the derailleur better than having the wheel move straight down. There are some pictures on Sheldon Brown's page.

http://www.sheldonbrown.com/gloss_dr-z.html#dropout

A horizontal dropout allows for chain tension adjustment on 3-speeds, single-speeds, and fixies. Hence old frames with horizontal dropouts are in demand for fixie conversions.