A number of Southwest’s aircraft wear distinctive special liveries, including 737-7H4 N918WN (c/n 29843), pictured here at Las Vegas, which has a livery called Illinois One.

Another colourful Southwest special is 737-7H4 N280WN (c/n 32533), seen at Fort Lauderdale Hollywood International, wearing its Missouri One livery.

Boeing 737-7H4 N778SW (c/n 27883) photographed at Denver International Airport. Southwest Airlines introduced a revised colour scheme in 2014.

Southwest 737s on stand at Los Angeles International. The airline claims 30% of the Los Angeles Basin, the USA's second-largest market, and is currently modernising its facilities at the airport in a project scheduled to be completed in the second quarter of 2018.

Southwest flies the world’s largest all-Boeing 737 fleet - 724 aircraft in several iterations, including 737-8H4 N8643A (c/n 42524) pictured at Seattle - Tacoma.

One of the first Southwest 737 MAX aircraft, N8705Q (c/n 42558), at Boeing’s Renton facility in January 2017. Norwegian will be first to put the 737 MAX 8 into service, but with 200 737 MAX 8s on order, Southwest will be one of the major operators of the updated 737.