Originally Posted by
sillygolem
On single speeds and internal gear hub bikes this was done to adjust chain tension. The practice carried over to derailleur-equipped bikes: many early derailleurs clamp on to the dropouts. Slowly they moved off of the dropout onto their own bolt hole eliminating the need for horizontal dropouts. (70's 10-speeds are used a lot for single speed and fixie conversions because they have the dropouts and they're plentiful.) This didn't completely disappear until the late 90's or so on LBS bikes.
Thanks for the info. I have two other bikes (surly & dahon), none of them have this type of horizontal dropouts. It's a pain to mount wheels on such dropouts. The wheel just keeps falling and if you tighten the bolt, the axle keeps shifting to the right. I hope i dont get a flat on the rear wheel ever..lol.