Dropouts are where the axle of the rear wheel attaches to the bike frame proper.
They come in several verities. Chain Tensioners, Horizontal dropouts, Sliding dropouts, Eccentric bottom brackets, all solve the same problem, they take up chain slack.
Vertical dropouts, the most common verity, do not allow the rear wheel to be moved aft. Chain slack must be dealt with a chain tensioner.


Vertical Dropout (dropout images from Sheldon Brown's web site, check out his definition here

Horizontal dropouts allow the rear wheel to be moved aft.

Horizontal Dropout

Like a Horizontal, a sliding dropout allows the rear wheel aft movement, however it also has disk brake mounts, allowing the disk brake to stay aligned to its rotor.

Sliding dropout {image from silverfish-uk.com}

An EBB (eccentric bottom bracket) instead of moving the rear wheel aft, it moves the cranks forward.

Eccentric Bottom Bracket {Tom Teesdale}



Sorry about the crappy image, used my phone for the camera.
The chain tensioner I use is a little different than the norm. Mine has only one wheel. It functions fine for an internal hub or single speed, but would not work with a derailleur.

Another kind of dropout you may encounter is called a track dropout. Technically they are referred to as "Track bike fork ends" and not dropouts.


The more common verity of chain tensioner.

Hope this is of some help,

--A