The seas have historically performed two important functions: first, as a medium of communication, and secondly as a vast reservoir of resources, both living and non-living. Both of these functions have stimulated the development of legal rules. The fundamental principle governing the law of the sea is that ‘the land dominates the sea’ so that the land territorial situation constitutes the starting point for the determination of the maritime rights of a coastal state.

The seas were at one time thought capable of subjection to national sovereignties. The Portuguese in particular in the seventeenth century proclaimed huge tracts of the high seas as part of their territorial domain, but these claims stimulated a response by Grotius who elaborated the doctrine of the open seas, whereby the oceans as res communis were to be accessible to all nations but incapable of appropriation. This view prevailed, partly because it accorded with the interests of the North European states, which demanded freedom of the seas for the purposes of exploration and expanding commercial intercourse with the East.

The freedom of the high seas rapidly became a basic principle of international law, but not all the seas were so characterised. It was permissible for a coastal state to appropriate a maritime belt around its coastline as territorial waters, or territorial sea, and treat it as an indivisible part of its domain.