Many travelers like to board first to make sure there is room in the overhead compartments to store their carry-on (except for those who believe that priority boarding is actually bad). And when it comes to boarding first on Southwest , you’ll have to approach your flight differently than if you were flying on Delta, United, or any other major airline.

There is one simple reason: Checking in as early as possible leads to a better boarding group.

For the unfamiliar, Southwest uses a different boarding process than other major airlines. Passengers get boarding groups (either A, B, or C with a subsequent number from 1-60 of where in that boarding group they stand) instead of assigned seats. Those boarding groups are, for the most part, assigned based on when passengers check in. The better your boarding group, the better chance a passenger has of finding overhead bin space and snagging an aisle or window seat.

Scott Keyes, founder of Scott’s Cheap Flights, told Travel + Leisure that Southwest is one of the few airlines where it actually matters how early you check in because seats are first-come, first-serve based on check-in time. Like other airlines, check-in starts 24 hours before the flight for Southwest. Keyes notes that even waiting five to 10 minutes after check-in opens could mean getting bumped to the B or C seating groups (which means a lower likelihood of sitting with your group or getting an aisle or window seat).

Southwest credit cards come with four upgraded boardings per year, so your position may depend on who is using their perks on your flight regardless of how early you check in. If there are any passengers on flights that don’t require a plane change, they will already have their seat as well (Southwest doesn’t kick them out after they spend the first leg of a trip in a certain spot, after all). There are also Southwest A-List and A-List Preferred flyers who get their boarding number before the rest of the passengers can check in.

Southwest does have a program called Early Bird Check-in that gets around the whole issue of setting an alarm to check into your flight the second opens — to a point. It’ll guarantee that you’re among the first to board if you pay between $15 and $25 per flight (meaning you pay twice for a round-trip booking).

However, Early Bird Check-In doesn’t jump ahead of the 15 priority boarding spots reserved people who buy Business Select tickets (meaning the A 1-15 spots).

Still, if you want to have the best chance possible of getting the seat you desire on your Southwest flight, you might want to follow Keyes’s advice and set an alarm for when check-in opens.