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Rules concerning the Control of Wireless Telegraphy in Time of War and Air Warfare. Drafted by a Commission of Jurists at the Hague, December 1922 - February 1923.
Part II: Rules of Air Warfare#Chapter I: Domain of application: classification and marks. - Art. 7.
Art. 7. The exterior marks required by the above articles
shall be affixed in such manner as to make it impossible for them to be altered during flight. They shall be as large as possible and shall be visible from above, from below and from either side.