C9, C09 or C-9 may refer to:* C9, an international technology based solutions provider C9 Operations based in Pompano Beach, FL. C9, an international standard paper size (40×57 mm), defined in ISO 216. C9 engine, by Caterpillar Inc. C9, a global group of nine leading economists based in Zürich, Switzerland C9, an extremely high "musical" note five octaves above Middle C. Only Georgia Brown (Brazilian singer) (not even Mariah Carey, Minnie Riperton or Adam Lopez)'s reputed to sing t… (More on C-9)