Gate card: Allegiant Air (G4)

The airline's policy: “Passenger of Size (under 'Passengers with Special Needs')”

Allegiant's seats measure 17.8 inches between armrests. If you cannot lower the armrest and/or you would compromise any portion of the adjacent seat(s), Allegiant says you should purchase an additional ticket during the initial reservation. When you buy the second seat, two side-by-side seats are pre-assigned at no additional charge. If you wait until the day of travel, an agent can only sell a second ticket if two adjacent seats are still available; if the flight is sold out, the passenger 'shall be denied travel in the interest of safety.'

Refunds: Not published by Allegiant. Secondary sources (e.g., SmarterTravel airline policy roundups) report Allegiant does NOT refund the second seat even if the flight has open seats — unverified on the airline's own site.