On fixed-wing aircraft, marks must be displayed on either the
vertical tail surfaces or the sides of the fuselage. If displayed
on the vertical tail surfaces, they shall be horizontal on both
surfaces of a single vertical tail or on the outer surfaces of a
multivertical tail. If displayed on the fuselage surfaces, then
horizontally on both sides of the fuselage between the trailing
edge of the wing and the leading edge of the horizontal
stabilizer. Exceptions to the location and size requirement
for certain aircraft can be found in 14 CFR part 45.
On rotorcraft, marks must be displayed horizontally on both
surfaces of the cabin, fuselage, boom, or tail. On airships,
balloons, powered parachutes, and weight-shift control
aircraft, display marks as required by 14 CFR part 45.
Almost universally for U.S.-registered, standard certificated,
fixed-wing aircraft, the marks must be at least12 inches high.
A glider may display marks at least 3 inches high.
In all cases, the marks must be of equal height, two-thirds
as wide as they are high, and the characters must be formed
by solid lines one-sixth as wide as they are high. The letters
“M” and “W” may be as wide as they are high.
The spacing between each character may not be less than onefourth
of the character width. The marks required by 14 CFR
part 45 for fixed-wing aircraft must have the same height,
width, thickness, and spacing on both sides of the aircraft.
The marks must be painted or, if decalcomanias (decals),
be affixed in a permanent manner. Other exceptions to the
size and location of the marks are applicable to aircraft with
Special Airworthiness certificates and those penetrating
ADIZ and DEWIZ airspace. The current 14 CFR part 45
should be consulted for a complete copy of the rules.