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 1996 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.