This paper, focusing on the Portuguese Cartaz system, attempts to trace the historical context of control of the seas. Control of an ocean area
can be defined as the capacity to cruise therein and also to hinder others
from doing so. In practice, it consists chiefly in the material, if not
legal, ability to seize vessels belonging to other parties and to avoid
seizure of one's own vessels by the latter. From the beginning of the
sixteenth century, the Portuguese claimed suzerainty over the Indian
Ocean, permitting sailing only to purchasers of the Portuguese Cartaz.
The foundation of this practice was the sale of protection. Tribute
demanded from Asian traders and their ships took the form of the Cartaz
system. Ships in the Indian Ocean sailing to a destination not reserved
by the Portuguese for their own trade had to buy a safe conduct pass to avoid seizure and confiscation of merchandise. Related to the Cartaz
was the introduction of coercion and monopoly by the Portuguese in sixteenth century Indian Ocean