"It is not entirely clear why N was originally chosen for the United States when U and US were available and would seem more logical choices (A for America might also be a good choice but was used for Austria for some period of time). The prevailing theory as to why N was chosen dates back to the days of Morse code when an international conference agreed to identify various nations by a one-letter designation to avoid confusion. Since the US Navy had already been using N to identify its stations, it is believed that this letter was chosen to represent the entire United States. Many of these wireless communication codes were later adopted for aviation as well, so the N was carried over and has been retained to the present day."
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