American flys out out of multiple concourses. The first letter of your gate will tell you which concourse. The number that proceeds is your gate number within that concourse. The underground train will take you to your appropriate concourse. There is also a walkway if you have the time and want some exercise.
ATL is different than most airports, and is the easiest to navigate. There is just one Domestic terminal with a South and North side. The South side is just Delta, North is all others, including American. From this terminal you take an underground train that delivers you to one of 6 domestic concourses,A-E, in line. Each concourses has a number of gates (more than any other airport). If you are traveling internationally, there is a separate international terminal with a separate drop off area, parking, baggage check / claim and security line. Once past security, It is also connected to the same underground train from domestic, and is the last stop from the domestic terminal.
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