Many carriers have a "first come, first serve" seating policy meaning that customers can sit anywhere so long as they get to the gate early and are one of the first ones in line. 

Others will provide customers with numbers for their seats and, if you book tickets together, as in 2-3 at a time, the carrier will most likely have your passenger numbers in sequence so that you sit close to your friends and/or family. 

Other carriers will allow you to call in to change to select a specific seat. Additional fees may apply. 

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