Who are the big four US airlines?

When people say the "big four," they mean the four airlines that move most of the country: American Airlines, Delta Air Lines, United Airlines, and Southwest Airlines. If you've flown domestically in the US, there's a very good chance you were on one of them. Every other carrier, from JetBlue to Alaska to Spirit, competes for what's left.

The four airlines, quickly

• American Airlines: The result of a 2013 merger with US Airways. Big hubs at Dallas/Fort Worth, Charlotte, and Miami.
• Delta Air Lines: Known for reliability and a strong loyalty program. Major hubs in Atlanta, Detroit, Minneapolis, and Salt Lake City.
• United Airlines: Strong international network. Hubs in Chicago, Denver, Houston, Newark, and San Francisco.
• Southwest Airlines: The country's largest low-cost carrier, famous for point-to-point flying and, for years, two free checked bags.

How much of the market do they hold?

A lot. Depending on how you measure it, by seats flown, passenger miles, or total passengers, the big four together control somewhere in the range of two-thirds to three-quarters of US domestic air travel. Individually, each one tends to sit somewhere in the mid-to-high teens by percentage, so no single airline runs away with it, but as a group they dominate. That concentration is why fares, routes, and baggage policies at these four carriers shape the whole travel experience for most Americans.

Why these four and not others

Three of them, American, Delta, and United, are called "legacy" or network carriers. They fly huge hub-and-spoke networks, belong to global alliances, and connect you almost anywhere in the world. Southwest is the odd one out: it grew up as a low-cost airline flying shorter point-to-point routes and doesn't belong to a global alliance. It earns its spot in the big four purely on size, because it carries a huge number of domestic passengers.

What this means for you

For most US trips, comparing the big four will cover the majority of your realistic options, but it's worth checking a smaller carrier too. Airlines like Alaska, JetBlue, Frontier, Spirit, and Allegiant often serve specific routes at lower prices or with a nonstop the big four don't offer. A quick rule: use one of the big four for wide coverage and loyalty perks, then price-check a low-cost carrier before you book. You might save real money on the exact route you need.