Originally Posted by Retro Grouch

If your bike has a braze on for the front derailleur, that's what you have to get.

If your bike doesn't have one you can either buy a clamp on front derailleur or use a braze on front derailleur along with a separate clamp.

Clamp-ons generally give you a little more freedom to locate your front derailleur a little bit higher or lower. That can be useful if you decide to install a compact crankset onto a bike that was designed for a traditional crankset.