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Declaration concerning the Laws of Naval War. London, 26 February 1909.
Chapter I : Blockade in time of war - Art. 9.
Art. 9. A declaration of blockade is made either by the blockading Power or by the naval authorities acting in its name.
It specifies --
(1) The date when the blockade begins;
(2) the geographical limits of the coastline under blockade;
(3) the period within which neutral vessels may come out.