The DC-9 aerodynamic design features discussed include the aft-engine T-tail arrangement, wing planform, airfoils, high-lift systems, nacelle-pylon design, and aerodynamic control systems. The aft-engine design is shown to have distinct advantages for the DC-9. There are two DC-9's : the Series 10, without leading-edge devices and therefore with airfoils having exceptional maximum lift capability; and the Series 30, with a longer fuselage, leading-edge devices, and airfoils more oriented toward minimum high-speed drag. The discussion concentrates on design areas associated with the aft-engine T-tail configuration such as the solution of the deep-stall problem and the design of the nacelle-pylon-fuselage area to minimize high Mach number drag. Causes of "locked-in" deep stall are shown to be related to the combined effects of wing and nacelle wakes and fuselage vortices on the horizontal tail. The DC-9 configuration has enough nose-down pitching moment capability to eliminate the possibility of locked-in deep stall. An analog simulator was used successfully to interpret extensive deep-stall wind-tunnel data. Flight-test results in all flight regimes verified the predictions of analytical methods and wind-tunnel tests.