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Post mount (PM) is a standard of brake mounts. Adapters are used to suit larger rotors. When I go to a shop, I see that apparently there are versions labelled as front or rear specific. On the other hand I see models which seem to work for both. This at least applies to shimano mount adapter.
Are disc brake post mount adapters always compatible for front and rear? Or if not, why are there differences?
I actually can answer it now myself. Shimano has "rear" and "front" adapter. This is a relict from the time where post mount adapters were not used yet on the frame and therefore post mount were only for the front. See this chart from Shimano Other brands like Avid/SRAM don't even distinct between front and back.
Post mount adapters are in general compatible for front and back, making the front/rear label obsolete.
Source:http://forums.mtbr.com/brake-time/rear-shimano-adapter-160-203-pp-can-i-use-front-mount-1083866.html
