Legacy air carriers of the United States

A mainline flight is a flight operated by an airline's main operating unit, rather than by regional alliances, regional code-shares, regional subsidiaries, or wholly owned subsidiaries offering low-cost operations. Mainline carriers typically operate between hub airports within their network and on international or long-haul services, using narrow-body and wide-body aircraft. This is in contrast to regional airlines, providing feeder services to hub airports operating smaller turboprop or regional jet aircraft, or low-cost carrier subsidiaries serving leisure markets.

Northwest Airlines (often abbreviated as NWA) was a major airline in the United States that operated from 1926 until it merged with Delta Air Lines in 2010. The merger made Delta the largest airline in the world until the American Airlines–US Airways merger in 2013.

United Airlines, Inc. is a major airline in the United States headquartered at 233 South Wacker Drive in the Willis Tower in Chicago, Illinois. United operates an extensive domestic and international route network across the United States and all six inhabited continents primarily out of its seven hubs, with Chicago–O'Hare having the largest number of daily flights and Denver carrying the most passengers in 2023. Regional service is operated by independent carriers under the brand name United Express.

North Central Airlines was a local service carrier, a scheduled airline in the Midwestern United States. Founded as Wisconsin Central Airlines in 1944 in Clintonville, Wisconsin, the company moved to Madison in 1947. This is also when the "Herman the duck" logo was born on Wisconsin Central's first Lockheed Electra 10A, NC14262, in 1948. North Central's headquarters were moved to Minneapolis–St. Paul in 1952.

National Airlines was a trunk carrier, a scheduled airline in the United States that operated from 1934 until it merged with Pan Am in 1980. For most of its existence the company was headquartered at Miami International Airport, Florida. At its height, National Airlines had a network of "Coast-to-Coast-to-Coast" flights, linking Florida and Gulf Coast destinations such as New Orleans and Houston with cities along the East Coast as far north as Boston as well as with large cities on the West Coast including Los Angeles, San Francisco and Seattle. From 1970 to 1978, National, Braniff International Airways, Pan American World Airways (Pan Am) and Trans World Airlines (TWA) were the only U.S. airlines permitted to operate scheduled passenger flights to Europe.

Piedmont Airlines was a United States local service carrier, a scheduled carrier that operated from 1948 until it merged with USAir in 1989. Its headquarters were at One Piedmont Plaza in Winston-Salem, North Carolina, a building that is now part of Wake Forest University.

Frontier Airlines was a United States local service carrier, a scheduled airline that was formed by the merger of Arizona Airways, Challenger Airlines, and Monarch Air Lines on June 1, 1950. Headquartered at the now-closed Stapleton International Airport in Denver, Colorado, the airline ceased operations on August 24, 1986. A new airline using the same name was founded eight years later in 1994.

Ozark Air Lines was a local service carrier (originally known as a feeder airline) in the United States that operated from 1950 until 1986, when it was purchased by Trans World Airlines (TWA). Ozark got a second chance to be an airline when the carrier that won the routes for which Ozark applied, Parks Air Lines, failed to start them in a timely manner. Parks had its rights revoked, Ozark won not only the routes it previously applied for, but others as well. Parks merged into Ozark and Ozark (then without any airline operations) took over the Parks operation and the single route over which Parks had recently started service, thereby launching Ozark. Ozark over time became a jet carrier with a hub in St. Louis.

Pan American-Grace Airways, also known as Panagra, and dubbed "The World's Friendliest Airline" was an airline formed as a joint venture between Pan American World Airways and Grace Shipping Company. On September 13, 1928, a small single-engine Fairchild airliner flew from Lima, Peru, to Talara, Peru, which marked not only the beginning of Pan American Grace Airways but also the inauguration of scheduled air transportation along the West Coast of South America. From this short flight in 1928 to nonstop flights from New York to South America with Douglas DC-8 Intercontinental Jets in 1966, Panagra became the standard-bearer for transportation between the US Mainland East Coast and the West Coast of South America for 39 years. The "World's Friendliest Airline" merged with Braniff International Airways in 1967, and the combined carrier became the largest US airline serving South America.

Texas International Airlines Inc. was a United States local service carrier, known from 1940 until 1947 as Aviation Enterprises, until 1969 as Trans-Texas Airways (TTA), and as Texas International Airlines until 1982, when it merged with Continental Airlines. It was headquartered near William P. Hobby Airport in Houston, Texas.