If you're a hamplanet, you have to purchase an additional seat if you spill over the armrest. They call it their "Customer of Size" policy:  Customers who encroach upon any part of the neighboring seat(s) may proactively purchase the needed number of seats prior to travel in order to ensure the additional seat(s) is available. The armrest is considered to be the definitive boundary between seats; width between the armrests measures 17 inches. The purchase of additional seats serves as a notification to Southwest of a special seating need, and allows us to adequately plan for the number of seats that will be occupied on the aircraft.  In turn, this helps to ensure we can accommodate all Customers on the flight/aircraft for which they purchased a ticket and avoid asking Customers to relinquish their seats for an unplanned accommodation. Most importantly, it ensures that all Customers onboard have access to safe and comfortable seating.  In 2010, Mallrats director Kevin Smith, 39, was kicked off a Southwest Airlines flight because "mah condishuns".