For passengers who don’t easily fit into the narrow seats on most commercial jets, it helps to know in advance what each carrier’s policies.

The folks at AirfareWatchdog.com recently put together a chart of more than a dozen North American airlines and their varying policies for larger passengers. We’ve pulled out the important information about five of the largest U.S. carriers:

United
United passengers need to demonstrate that they can put down the seat’s armrests and have them stay down. If this is not possible, they will be required to buy a second seat. It doesn’t matter if the passenger is seated next to a friend or family member. And if you don’t purchase that second seat at the same time as the original seat, you could be paying the more expensive walk-up fare at the gate.

Delta
Delta has no requirement for purchasing a second seat, but airline staff may request that you change seats or wait for a flight with available seats. Thus, Delta recommends that people who can not risk being bumped to a later flight buy a second seat at the time they book their original seat.

Southwest
Because of a 2010 incident involving film director Kevin Smith, Southwest has probably made the most headlines for either stopping passengers from boarding or requiring them to pay for a second seat. Southwest asks passengers who do not fit in the 17″ between the armrests to proactively book a second seat at the same price. Not doing so in advance could have them waiting for a later flight. If they book that second seat and then find out later that the flight was not entirely full, they can contact Southwest for a refund.

American
American asks customers who are unable to fit into a single seat, unable to properly buckle their seatbelt with an extender or unable to lower both armrests without encroaching to “address their seating needs when booking the original reservation.” Booking two adjacent seats in advance will get the passenger the same rate. Those who haven’t booked in advance may be able to score a free second seat by talking to the gate agent.

US Airways
US Airways doesn’t provide specific guidelines to passengers, but claims to deal with this issue on a case-by-case basis. When it can, the airline will offer extra space to passengers who require it, though they may be asked to wait for a flight with available seats. Passengers who need the extra space but can not wait for non-full flight will be required to buy an extra seat at the gate.

For the entire list, check out AirfareWatchdog.com

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