Southwest Airlines offers boarding option many passengers love

Booking flights on most major U.S. airlines requires already having a specific seat reserved before entering the aircraft.

This is not the case with Southwest Airlines  (LUV) , which has a different policy that allows passengers to select their seats as they board.

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Before boarding, travelers are assigned groups in which to board the airplane. For obvious reasons, if you are in a group that boards earlier, you have a wider selection of seats from which to choose.

Passengers are assigned to groups labeled A, B or C. In each group, they are also given a number that specifies the order to board in each group.

Group A boards earlier than Group B, which boards before Group C. Each of the carrier's aircraft have three seats to the right of the aisle and three to the left.

Passengers can check in for their flights using their devices 24 hours before departure. Generally speaking, the groups are assigned in the order the passenger checks in.

But, of course, it's a bit more complicated than that.

What it takes to secure early boarding

Southwest's preferred frequent flier program is called Business Select. Members who hold these tickets occupy a dedicated check-in lane that boards first and takes the first fifteen rows of seats.

"You can also sometimes upgrade (for a fee) to an A1-15 boarding spot 30 minutes to 24 hours before the flight's scheduled departure time," wrote Summer Hull and Jennifer Yellin for The Points Guy on Feb. 27. "This option is only available if Business Select customers don't fill the A1-15 slots, and it costs between $30 and $80 (per segment, per customer)."

There are a couple other categories for checking in early, but the main one available to economy passengers is Southwest's EarlyBird Check-In.

This service allows travelers to reserve boarding spots 36 hours before their flights.

"EarlyBird Check-In serves two obvious purposes. First, it eliminates the human element of forgetting to check in exactly 24 hours before your first flight," Hull and Yellin wrote. "Second, it scores you a boarding assignment that should be better than those available 24 hours before departure."

EarlyBird Check-in is priced at $15 to $25 per passenger traveling each way on a round-trip ticket.

Whether paying for EarlyBird Check-In is worth the money

For some passengers, the order in which they board the airplane isn't of huge importance. This is particularly true for those who are looking for ways to save any money they can on travel.

Adults traveling with children are allowed to choose family boarding, which allows them to sit together. Family boarding occurs after Group A boards and before Group B boards.

"In my experience, if you're eligible for family boarding, there is absolutely no reason to purchase EarlyBird Check-In as your family will have no problem sitting together," wrote The Points Guy.

"If you have the budget to consider buying EarlyBird Check-In, it might be worth it in a few cases," the authors continued. "First, EarlyBird Check-In might be worth it if the flight is especially long, such as a Southwest flight to Hawaii. The length makes seat selection more valuable."

It is also possible to purchase EarlyBird Check-in for one leg of a trip and not the other, so there are options for travelers on a budget.

"At $15 to $25 per person each way, Southwest EarlyBird Check-In can be worth the cost," wrote Hull and Yellin. "However, it won't always be a slam-dunk deal — especially if family boarding is an option for your group."

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