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When you booked the tickets does not matter for mileage accrual - what matters is that you are a member of the frequent flyer programme when you actually do fly.

If you sign up for AAdvantage, or any other program, you will receive a frequent flyer number. If you then call the airline or travel agent that booked your ticket, they can add the number to your booking file. Also, make sure to show the number at check-in for each flight, so the agent can verify that it is entered correctly in the system.

If you give us some details of your route and which airlines you will be flying most on your trip, we can try to give you some insight into whether American Airlines offers the right programme for your needs or if another oneworld carrier has a better option.