The United States uses the letter "N" as a prefix for aircraft registration numbers, a practice that dates back to the International Air Navigation Convention of 1919. This convention established a system where each country was assigned a unique identifier for its aircraft. The U.S. was designated with the letter "N," which has been used consistently since then.
According to the Federal Aviation Administration (FAA), "The U.S. received the 'N' as its nationality designator under the International Air Navigation Convention, held in 1919." Initially, this designation included radio letters N, W, and some combinations of K (KDA to KZZ). The choice of N became permanent due to its early use by the US Navy in November 1909.
In March 1927, amendments to the Air Commerce Regulations mandated that all U.S.-registered aircraft must begin their registration numbers with an N-prefix. Over time, specific guidelines have been developed regarding how these codes are formed and displayed on aircraft.