14 CFR 45.23: Display of marks; general.

(a) Each operator of an aircraft must display on that aircraft marks consisting of the Roman capital letter “N” (denoting United States registration) followed by the registration number of the aircraft. Each suffix letter used in the marks displayed must also be a Roman capital letter.

(b) Except for unmanned aircraft, when marks include only the Roman capital letter “N” and the registration number is displayed on limited, restricted, experimental, or provisionally certificated aircraft, the operator must also display on that aircraft near each entrance to the cabin, cockpit, or pilot station, in letters not less than 2 inches nor more than 6 inches high, the words “limited,” “restricted,” “experimental,” or “provisional,” as applicable.