C9, C09 or C-9 may refer to: Biology, medicine, and chemistry C9 (Complement component 9), a protein ATC code C09, a subgroup of the Anatomical Therapeutic Chemical Classification System C09, ICD-10 code for malignant neoplasm of tonsil Carbon-9 (C-9 or 9C), an isotope of carbon Military and weapons Hi-Point Models C9 and C9 Comp handguns C9 LMG, Canadian light machine gun C9, an ID for the German Nachtjagdgeschwader 5 air squadron in World War II Music C9, a note five octaves above Middle C C9, a C ninth chord Organizations Cloud9, an American esports organization C9 League, an association of Chinese universities The Council of Cardinal Advisers, an advisory body to the pope, originally comprising nine members C9 Entertainment, a South Korean entertainment company and record label Transportation Cierva C.9, a 1927 British experimental autogyro HMS C9, a British submarine Ford C-9, a US military designation for the Ford Trimotor aircraft McDonnell Douglas C-9, a US Air Force transport aircraft based on the civilian DC-9 USS Montgomery (C-9), a US Navy cruiser C9, the IATA code for Cirrus Airlines Sauber C9, a Le Mans racing car C9 engine, by Caterpillar Inc. C-9 (Cercanías Madrid), a commuter rail line in Madrid LNER Class C9, a class of 2 British steam locomotives rebuilt from C7s in 1931 Other uses C9, an ISO 216 standard paper size C9, a holiday light bulb size C9, a sportswear line by Champion See also 9C (disambiguation)