The Boeing 737 Next Generation, often called the 737NG, is a family of Boeing 737s. The 737-600, -700, -800 and -900 are all 737NG aircraft. It is the third family of Boeing 737. The family which came before it is the 737 Classic (−300/-400/-500) family. They have been made since 1991 by Boeing Commercial Airplanes. All 737s are twin-engined, narrow-body airliners. This means that they all have two engines and only have one aisle between seats.
4,293 737NG aircraft have been delivered (as of December 2012). More than 6,300 have been ordered by airlines. Southwest Airlines has more 737NGs than any other airline. Ryanair, a low-cost airline from Ireland, also has a lot of 737NGs. The only planes Ryanair has are 737-800s. The main rival of the 737NG family is the Airbus A320 family. The Boeing 737 MAX will eventually replace the 737NG.