Even as other airlines cracked down on weight limits and raised the costs of checking bags on shorter flights, Southwest used this generous policy to build out a loyal customer base. "Two bags fly free" is formally a registered trademark with the airline.

Southwest Airlines will begin charging customers a fee to check bags, abandoning a decades-long practice that executives had described last fall as key to differentiating the budget carrier from its rivals.

Customers with the highest loyalty status will be spared from the fee, but others will have to pay for checked bags for flights booked from May 28.