United Airlines assigns seats; rival Southwest Airlines does not. That distinction is the basis of United’s new digital ad campaign that pokes fun of Southwest’s “C” boarding group—the last travelers to board its planes.

Just in time for the holiday travel season, the Chicago-based United rolled out a series of digital ads this week that direct viewers to “NotGroupC.com.” There, travelers are invited to sign up for a call that will remind them to check in for Southwest’s C group—and also tout United’s assigned seat program. If Southwest travelers don’t remember to check in exactly 24 hours before their flights, they are often stuck in the last boarding group.

“We’re sounding a light-hearted alarm to remind travelers who may not choose United that there’s an easier way to travel,” said Maggie Schmerin, managing director and head of global advertising and social media at United, in a statement. “Ideally, the next time these travelers are flying, they won’t need to check in 24 hours before or risk being in Group C, because they will have chosen to fly with United and not one of our competitors.”

News of the campaign was first covered by travel blog the Points Guy.

Ads directing people to NotGroupC.com will run on social media and digital out-of-home in cities including Denver and Chicago, where United and Southwest compete. United worked with its agency of record, 72andSunny, on the quirky campaign.

Southwest Airlines did not immediately respond to a request for comment.

United has found success in zigging while others are zagging, according to Schmerin. The airline used pandemic lockdowns to reinvest in its brand, overhauling merchandise, employee training and marketing, in order to emerge on top, she shared at the Association of National Advertisers’ Masters of Marketing conference last week. Its “Good Leads the Way” brand marketing campaign, released earlier this year with 72andSunny and Dentsu, have helped increase brand perception for United, Schmerin said at the conference.

Last month, United reported third-quarter results that exceeded analyst expectations. Total operating revenue grew 13% to $12.9 billion. Net income, at $942 million, nearly doubled that of the year-earlier period. Southwest, meanwhile, reported third-quarter operating revenue of $6.2 billion and net income of $277million.