Why do all us air planes use the letter N in their ID?

All U.S. airplanes use the letter "N" in their identification because it designates American registered aircraft in accordance with the International Civil Aviation Organization (ICAO) standards. The "N" prefix is part of the U.S. aviation regulations, which require that all civil aircraft registered in the United States carry a registration number starting with this letter. This system helps in the identification and regulation of aircraft within U.S. airspace.