Dark blue flag with a white emblem. The emblem consists of a compass rose (and lines reaching out from it). Dimensions: 2:3, 3:4 and 1:2.
Member states (with dates of joining):
Czech Republic (1999)
United Kingdom (1949)
United States of America (1949)
The NATO flag is 3:4, as described at the Nato Website.
I quote a portion of the above referenced web site:
The Official NATO-color code is Pantone Color Guide No 280
Dimensions Length: 400 Width: 300 Star: 150 Diameter of circle 115 Space between points of star
and start of white lines
10 Space between outer edge of flag
and extremity of white lines
A short note on this subject appeared in Recueil de l'Office Généalogique et Héraldique de Belgique III, Editions Tradition et Vie, Brussels, MCMLIV (1954), 120 p. plus card. A note on p. 72 'Le drapeau et l'emblème de l'OTAN' (The flag and the emblem of NATO) written by A. de Selliers de Moranville said the card shows the NATO flag. According to this note, the shade of blue corresponds to French norm Pr X.08-002 (AFNOR) and to No. 218 of the British Colour Council. Nowadays NATO officially mentions Pantone 280.
My Observer's Book of Flags of 1959 says the dark blue represents the Atlantic Ocean, the compass rose indicates that NATO's work is directed towards world peace, and the white circle signifies unity.