Southwest Airlines uses a unique seating and boarding process where nobody is assigned seats, but rather you select your seat as you board the plane. Boarding order is determined by the time you checked-in for your flight, but if you’re especially picky you can pay extra to get a guaranteed top 20 selection. This seating policy means that the plane gets boarded more quickly since people are standing around waiting for access to their specific seat, but just move to an open seat they want. It also means you can choose window or aisle seat on the spot rather than being stuck with what you chose when you bought your ticket.