Straits are narrow stretches of water between two land masses joining two marine expanses. A strait (or also the plural, straits) can connect two oceans (the Torres Straits, the Straits of Malacca), an ocean to a sea (the Straits of Gibraltar), two «continents» (the straits of Gibraltar, the Bosphorus, or the Behring Straits), an island and a continent (Straits of Malacca, Formosa Straits, Dover Straits), or two islands (Bonifacio Straits and the straits in the Philippines and Indonesia).
In marine legislation, there is a distinction between an interior strait and an international strait. A strait is international when it links one high seas area to another, or to a territorial sea belonging to a foreign state, and providing it is used for international circulation. Straits that are used solely for internal navigation purposes by a state, or that are little used, are therefore considered to be domestic. The status of straits can change over time: with the melting of the ice caps and global warming, Canadian and Russian straits in the Arctic could become international straits.
In both cases, although rather differently, this raises the question of national integration. A strait is a line of discontinuity, a territorial divide that numerous states seek to remove by stepping up maritime connections (ferries, container transport, ro-ro ferries etc.) or by building fixed links (bridges and tunnels). In an international strait the question of national sovereignty is more delicate – even of it is inside one state’s territorial waters, the neighbouring states are all required to apply principles of free maritime circulation, and to observe the Montego Bay Convention of 1982: free passage for naval ships and fleets, no tolls, and obligations in terms of safety and security for navigation. It should also be said that many straits define «borders,» since they form, de facto, a physical boundary in the same way as do rivers or mountain ranges. When two states are separated by an expanse of water less than 24 miles across, the straits are generally shared out using a rule of equal distances from the dividing line. However a maritime border is distinct from a land border, since it cannot be reduced to a line, it has a certain width and also a depth. It is possible to cross it, but it can also be followed lengthwise. In addition the sea is not a void, and despite rivalries or political conflicts, neighbouring states are obliged to take it into account, and sometime to collaborate in the management of this fragile environment: pollution risks, over-fishing, destruction of ecosystems. The proximity of the coastlines make the unilateral management of maritime resources unfeasible, and environmental risks are shared despite the existence of an international border.
This particular, complex space forms a maritime and terrestrial interface, and to study a strait we need to consider longitudinal flows, cross-flows and the maritime space per se, since it at once constitutes the dividing line and the zone of contact in either direction.
A strait is a maritime corridor through which there are longitudinal flows. Because of this, international straits are crucial in international trade. They are ocean doorways, and sometime the only access route, and they channel ever-increasing volumes of global exchanges, and favour the establishment of large ports on their coastlines. At the same time they are the “weak link” in maritime routes, bottlenecks where conditions of navigation are often difficult. Straits are organised like maritime motorways – there is a system of traffic separation with shipping lanes parallel to the coastlines, differentiating the two directions of traffic. While straits serve to link, they are also easier to control than the high seas, and this enables the maritime traffic to be controlled. The possibility of straits being closed constitutes an economic threat, and also a military threat, since it prevents the deployment of naval fleets. The controlling of access to international straits is a strategic and geopolitical issue, both for the bordering states and for the main powers. Straits are vulnerable spaces, and insecurity or unrest in the bordering areas extends to the maritime area.
Straits also form points of crossing between one «littoral» and another, between two countries, or two continents. Although it is a barrier for circulation on land, it affords greater crossing opportunities that elsewhere because the two side are particularly close. From a terrestrial viewpoint, the strait is a crossing-point where the crossing is the shortest possible. This “bridge effect” shows up in the concentration of all terrestrial flows of people and goods to these particular spots. Thus a strait is a transit space, and when a fixed link is set up between the two shores, (like a tunnel, but also gas pipelines, electricity or telecommunication cables) and when the missing terrestrial link is no longer missing, it can become a crossroads hub for terrestrial and maritime transport.
In a strait, does the sea occupy a peripheral or a central position? This depends on the viewpoint adopted, whether maritime or terrestrial, and also on the degree to which the strait is open to cross-traffic. When a strait defines a closed border, or when the two coasts and their populations are face to face or back to back, the sea is in a peripheral situation in relation to the bordering states. If the strait has international status, it may even be seen as a weakness in the national system of defence. Conversely, when the border is open, and there are crosswise flows linking the two shores, the maritime space is central, and it can be likened to an inland sea. The strait, its two shores and its maritime space can then be envisaged as a genuinely functional geographical entity. Whether it is viewed as a terraqueous space, meaning that it of formed of land and water, or a transborder space if is serves as a boundary, the understanding of this territory needs the interactions occurring on this land-sea-land interface to be explored, with its openings at once towards other maritime spaces and towards the continental hinterlands.