How low can you go? And what are you willing to sacrifice to go so low? Those are the questions posed by bare fares.

Bare fares are what cheeky ultra-low-cost carrier Spirit calls its stripped-down airfares: extra-cheap published ticket prices with few perks and plenty of restrictions that force travelers to pay extra for seat assignments, carry-on bags, and so on and so on. It’s a business model that flyers love to hate, but one that has made Spirit solidly profitable and a Wall Street darling.

Of the Big Three legacy airlines, Delta was the first to embrace the concept, in 2012 introducing Basic Economy fares to compete with Spirit on routes where the two carriers competed.

Today Delta offers Basic Economy on 40 percent of its domestic routes, and expects to expand coverage to its entire domestic network before the end of 2017.

This week, the two remaining Big Three airlines, American and United, put their own versions of unbundled fares on sale in select markets.

On Monday, United introduced its Basic Economy fares on flights between Minneapolis/St. Paul and its seven U.S. hubs (Chicago O’Hare, Denver, Houston, Los Angeles, New York/Newark, San Francisco, Washington Dulles).

The Minneapolis rollout is just the beginning: Eventually, United’s Basic Economy will be offered across the U.S., the Caribbean, and on shorter Latin America flights.

And a day later, on Tuesday, American customers could book Basic Economy fares for travel beginning March 1 on 10 routes, from four of its hub airports in Charlotte, Dallas, Miami, and Philadelphia.

As will United, American will expand its Basic Economy availability “in line with customer interest.”

So by the end of this year, unbundled fares will be widely if not universally available for U.S. domestic flights. For now, Delta’s are the most customer-friendly, as theirs don’t restrict carry-ons or elite miles. But it’s highly likely, given the nature of competition in the industry, that Delta will soon adopt the harsher version of Basic Economy, to match American and United.

Travelers are always suckers for low prices, and that’s what Basic Economy is all about. But it’s also about restrictions, that can make for a less comfortable flight experience, and surcharges, that can make the final price a shocker.

Caveat emptor.

Reader Reality Check

Basic Economy: Problem or solution?

After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.