Direct Attach (Conventional)
The most common form of rear derailleur interface, Direct Attach enables a rear derailleur to be mounted to a derailleur hanger which typically uses an M10 x 1mm thread. The derailleur hanger will have a small tab that protrudes just below the M10 mounting hole. The B-limit screw of the rear derailleur rests on this tab and adjusts it's position relative to the cassette or freewheel.
Most derailleur hangers are replaceable these days, however; some inexpensive frames will still have an integrated rear derailleur hanger. Currently there are no common standards for how a derailleur hanger interfaces with a bicycle frame so almost every model of bicycle in the world uses a different derailleur hanger; this means there are literally thousands of different shapes and sizes. The rear derailleur is one of the most exposed parts of a bike so it can easily get bent so being able to align or replace it is important for proper gear shifting. If a frame with an integrated rear derailleur hanger is really bent or needs to be replaced the entire frame may be rendered useless since it can't be replaced.
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