If you are checking a bag, you must go to the airline's check-in counter... even if you have already checked in online and printed your boarding pass. Make sure that if you print the boarding pass at home (or if you are using a mobile boarding pass on your phone), that the barcode/QR code is properly displayed. Otherwise, your boarding pass will not work and the airline counter will have to print you one. That has happened to me a few times.
Most airlines will allow you to check in online, and print your boarding pass at home. Otherwise, just go to the check-in counter of the airline that operates your flight. The counters are on the departures level.
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