Seth Stevenson unpacks the secret of Southwest Airlines’s success, compared to other airlines which have been struggling to carry a profit:

While other airline fleets can employ 10 or more types of aircraft, Southwest uses just one, the Boeing 737. As V.P. of ground operations Chris Wahlenmaier explained to me, this results in all manner of cost-saving efficiencies: “We only need to train our mechanics on one type of airplane. We only need extra parts inventory for that one type of airplane. If we have to swap a plane out at the last minute for maintenance, the fleet is totally interchangeable—all our on-board crews and ground crews are already familiar with it. And there are no challenges in how and where we can park our planes on the ground, since they’re all the same shape and size.”

Southwest also doesn’t assign seat numbers. Which means that if a plane is swapped out, and a new one’s brought in with a different seat configuration (even within the world of 737s, there can be some variations), there’s no need to adjust the entire seating arrangement and issue new boarding passes. Passengers simply board and sit where they like.

Lastly, Southwest doesn’t charge customers for a first checked bag. That means time is saved on every departure since these people aren’t trying to cram their overstuffed carry-on bags into overhead bins.