Airline Policies for Passengers Who Don't Fit in One Seat

Many airlines are now implementing policies for passengers that don't fit in a "normal-width" seat, which is typically 17 inches wide.  Some are requiring the purchase an additional seat, while other airlines are dealing with each passenger on a case-by-case basis.  In some cases, passengers have been refused boarding.  These are some of the current airline policies:

Air Canada
After a Supreme Court ruling, the airline must offer larger passengers a free seat, but only after receiving an approved doctors note. Those without medical approval may expect to be required to purchase a second seat at a cost to be determined by the airline.

Airtran
If a customer cannot lower the armrest, they will be required to purchase an extra seat. The airline will sell you the second seat at the lowest available cost.   If none are available, the passenger may not be able to fly.

Alaska
Passengers must fit within the 17 inches between armrests or purchase a seat in advance. Those who do not cannot be guaranteed boarding unless a second seat is purchased in advance. The second seat can be purchased at the airport only if there is still space available on the flight, and the lowest available fare at the time will be offered.

American
Customers who are unable to fit into a single seat, unable to properly buckle their seat belt with an extender or unable to lower both armrests without encroaching must purchase an additional seat when booking the original reservation. Two adjacent seats will be offered at the same rate in advance; if you have not handled this when you arrive at the airport, see an agent before proceeding to the gate if empty seats are available, you may be able to snag one for free.

Delta
Passengers are not required to purchase additional seats based on size, but you may be asked to move or wait for the next flight with additional seating space. The airline does recommend that you purchase an additional seat in advance if you think that you might need one and cannot afford to wait around for a flight with empty seats.

Frontier
If airline staff determines that you will not fit into one seat, they can require you to purchase another one. The seat will be offered at the lowest possible fare at the time of purchase.

JetBlue
The airline does not officially require the purchase of a second seat; armrests are 17.8 apart, which good news for many larger customers. Passengers do have the option to purchase a second seat, regardless of size, at the current fare offered.

Southwest
Customers who encroach on any part of a neighboring seat should book the needed number of seats prior to travel. The armrest is considered to be the definitive boundary and measures 17 in width. The seat will be offered at the same price as the first one at time of booking it is not recommended to wait until you arrive at the airport; Southwest is known for being particularly inflexible on this issue.

United
Armrests must be able to go down and stay down regardless if you are seated next to a friend or family member you must purchase a second seat. Those who decline to do so or upgrade to larger seats risk being refused at the gate. A second seat may be purchased for the same fare as the original seat provided it is purchased at the same time those who do not risk being charged walk-up fares later on.

US Airways
Handles it case-by-case, offering extra space when available, or may require waiting for a later flight. If the passenger will not change flights, they will be required to purchase a second seat at the gate.

Virgin America
Larger guests are asked to purchase two seats in advance upon initial booking; no specific policy regarding those who choose not to.

Some airlines will give refunds for the second seat purchased if the airplane does not fly full.  That is something you would want to check directly with the airline about.