What letter is at the beginning of the registration number of all non-military aircraft in the US?

An aircraft registration is a code unique to a single aircraft, required by international convention to be marked on the exterior of every civil aircraft. The registration indicates the aircraft's country of registration, and functions much like an automobile license plate.

In the United States, the registration number is commonly referred to as an "N" number, because all aircraft registered there have a number starting with the letter N. An alphanumeric system is used because of the large numbers of aircraft registered in the United States. An N-number begins with a run of one or more numeric digits, may end with one or two alphabetic letters, may only consist of one to five characters in total, and must start with a digit other than zero.