Originally Posted by hillrider

You can check the wiki to see if AA and DL have an interline agreement.

As far as fees on interline baggage, there are no rules about this; the Conditions of Carriage and International General Rules AA1 imposed on you are vague, and the DOT has not forced the airlines to provide any clarity on this subject.

The practice seems that at each check-in, whatever rules the (operating) carrier that checks you in apply to all of the flights that you're checking in for, even if you're switching (operating) carriers. This is irrespective of whether you're booked directly or as a codeshare, whose carrier's tariff is used for the pricing unit(s) for the segment(s) you're checking in for, which carrier issued your ticket, or anything else.

Since there are no rules and this practice is not part of your contract, this observed practice may or may not be universal and may change at any moment.

You can (and should) let the DOT know about this confusion and how it's affecting you at http://airconsumer.ost.dot.gov/escomplaint/es.cfm; they've started listening to voters again, and can't force the airlines to fix the issue until they know about it from voters.