The airline operates more Boeing 737s in its fleet than any other airline in the world; Southwest is often cited as an example of an airline streamlining operations by having only one type of aircraft. However, Southwest operated leased 727-200 aircraft during the late-1970s and again in the mid-1980s and subsidiary TranStar Airlines operated DC-9s and MD-80s during the mid-1980s. Southwest has been a launch customer for all three of the Boeing 737 variants it currently operates, and was the first airline to put both the Model 500 and next-generation Model 700 into service. Southwest has a mix of old and new aircraft with both its "classic" and "next generation" 737 aircraft.