dropout (thing)

The brazed, welded, glued on or occasionally pressed-in parts of a cycle frame to which the wheels are clamped or bolted. The front dropouts are generally a C shape, opening downwards, with a lug on top which is brazed (etc.) into the fork blade. Rear dropouts are similarly attached to the seat stay and chain stay (or whatever structures replace them in non-diamond frames) come in three flavours:

• Track pattern. The slot on these opens to the rear, such that in order to remove the wheel, it needs to be first moved forwards to de-tension the chain and drop it off the sprocket(s). Also commonly used on BMX bikes and single-gear kids bikes and on some mountain bikes, especially with rear suspension.
• Standard road pattern. These open forwards, facing slightly downwards. Higher range parts have a screw adjuster accessible from the rear so that the optimum position of the wheel can be set for quick wheel changes (assuming that you use a derailleur gear or some other chain tensioning device). Most are long enough to allow for fore and aft adjustments of wheel position to take up chain slack if you use a single gear, but some are a bit on the short side. The right-hand dropout often incorporates a standard pattern gear hanger to attach the rear derailleur mechanism.
• Vertical road pattern. On these the slot opens downwards, making it easier to drop the bike onto the wheel. Can only be used with derailleur gears because there is not enough scope for movement to tension the chain otherwise, and are also not very forgiving of imprecision in the construction of the frame; they do however allow for a much tighter clearance between the seat tube and the rear wheel, which is a bit of a fetish area for stiffness-obsessed framebuilders.