When pilots or air traffic controllers use the expression "Charlie Charlie" (often abbreviated as CC) during radio communications, it serves as an affirmative acknowledgment meaning "Correct" or "Yes." This convention originates from historical maritime and aviation communication codes—specifically the International Code of Signals established in the nineteenth century—where the letter "C" (designated by the spoken word Charlie) stood for affirmative confirmation. Pilots and controllers use it as a concise, standardized voice procedure to minimize ambiguity on crowded radio frequencies and confirm that instructions or information have been correctly understood.