Southwest Airlines will bring its notorious policy for large passengers to AirTran Airways starting in March.

The new policy will require those passengers -- whom Southwest delicately calls "customers of size" -- to buy a second seat if they are flying in AirTran's coach section.

As of March 1, AirTran will require the purchase of more than one seat for a passenger who "in the carrier's sole discretion, encroaches on an adjacent seat and/or is unable to sit in a single seat with the armrest lowered," according to the AirTran contract of carriage.

Before this, AirTran has not had a stated "customer of size" policy, but AirTran employees at the gate typically offer to sell a second seat or an upgrade for a fee, Southwest said.

It is an AirTran policy change that's coming as Southwest integrates the airline.