In 1984 Southwest introduced new interiors and was the launch customer for the Boeing 737-300. Southwest is the largest operator of 737s in the world. It has operated 737s solely, except for a brief period of 727s in the fleet in the late 80s. In 1984, Southwest was serving Denver which it pulled out of in the late 80s (returning in the early 2000s). SW left Denver because the inhospitable weather and congestion at Stapleton disrupted the airline's famed 20 minute turnarounds. Denver is only 1 of 3 cities SW has ever pulled out of in it's history. It often becomes the dominant carrier within a year of start up service to a particular destination.