The requirement to purchase two seats on a plane generally applies to "Passengers of Size" who cannot fit within the armrests of a single seat or who require a seatbelt extender and still encroach upon the adjacent passenger's space. Each airline in 2026 has a slightly different "Customer of Size" policy. For example, Southwest Airlines is famous for its inclusive policy: they encourage passengers who need extra space to purchase a second seat in advance, but they will actually refund the cost of the second seat after the flight, even if the flight was full. Conversely, airlines like United, American, and Delta typically require the passenger to pay for the second seat at the current fare rate if a second seat is deemed necessary by the gate agent. This policy is primarily in place for the safety and comfort of all passengers, ensuring that everyone has full access to their own seat area and that the seatbelt can be properly fastened. It is a recommended practice to call the airline's "Accessibility Desk" before booking to understand the specific refund or booking procedures for your carrier.