Southwest Airlines will begin charging customers a fee to check bags. It's a significant change for the budget carrier, which built years of advertising campaigns around its policy of letting passengers check up to two bags for free.

"Two bags fly free" is a registered trademark on Southwest's website. But its decision to about-face on what executives long cast as a sacrosanct passenger perk brings the largest U.S. domestic carrier in line with its rivals, which together generated $5.5 billion from bag fees last year, according to federal data.