How can you tell if you see a 737 classic or a next-generation 737? The 600/700/800 are longer than the 500/300/400 that they replace. Differences include increased wing chord and span, wing area being increased by 25%. Next-generation vertical fin is higher and has increased area, so has the horizontal stabilizer become larger. Another recognition-feature are the engine inlets. 737 classic series aircraft are flatter on the lower side on the engine inlet than the next-generation aircraft. The more round-shaped inlets can be seen on the Boeing 737-800 pictured above.