Designed to bend or break and save your derailleur when nasty accidents occur, the rear derailleur hanger is a small but very important piece of equipment.
Unfortunately it seems, no two are alike, or so it appears when it comes to replacing one following an accident. Many are black or silver, but some bike manufacturers make them to colour match frames. With there being so many options available, it is vital to check the bike make, model and year to ensure the correct one is purchased. There are two and three hole versions available in many shapes and sizes. Some older bikes have the hanger built into the frame and is not compatible with replacement hangers.
Front derailleur hangers, again are wide and varied. Many direct bolt to the frame but some frames do not have brazed fittings for a direct mount hanger, but frame adapters are available to take the hanger. When possible try and match your hanger by make, model and year of your bike to ensure the correct hanger is obtained.