For the 737 Classic and 737 Next Generation (737NG) variants, Boeing switched to the CFM56 engine, which was a more efficient high bypass turbofan, but consequently was much larger in circumference. Because the 737s wings were closer to the ground than in previous applications for the CFM56, several modifications were made including mounting the engine forward of the wing and mounting engine accessories to the sides, giving the engine nacelle a distinctive flat-bottomed shape. These modifications also limited the bypass ratio compared to versions of the CFM56 used on other aircraft.