You are not allowed to visit Pheasant Island, which lies near the Atlantic Ocean terminus of the French-Spanish border. But "it can easily be seen from the Joncaux bank, on the Bay Path," the Web site for the local tourist office [1] suggests, without a hint of irony.

For border enthusiasts, that feels like insult added to injury. They're condemned to contemplate the tiny eyot [2] from either bank of the Bidasoa, the border river that separates Hendaye in France from Irún in Spain [3]. And they can only wonder about the inscription on the gleaming white monolith that graces the island.

That monument commemorates the Treaty of the Pyrenees, concluded on the island in 1659. It fixed the Franco-Spanish border, following the mountain chain of that name as the natural boundary between the two countries [4]. But tiny Pheasant Island is more than the place where that treaty was signed.

For centuries, the island, less than an acre in size, was a favorite royal meeting place, often serving as a bridal exchange. In 1615, Louis XIII of France and Philip IV of Spain first met their wives - each other's sisters - on the island, after having married them by proxy [5]. Later that century, Pheasant Island would be the place where both Louis XIV of France and Charles II of Spain first laid eyes on their respective brides.

In recognition of its historical significance, Pheasant Island [6] is also known in French as Île de la Conférence [7]. The Treaty of 1659 consecrated its significance to both countries by establishing it as a rare example of that curious border arrangement: a condominium.

A condominium is a territory jointly administered by two or more countries, often (but not necessarily) a territory on the common border between the parties involved. As one might surmise, such an arrangement depends on the benevolent cooperation of all parties involved - and indeed, historically, most condominiums have not survived very long.

Pheasant Island is not only the oldest surviving condominium, it is also the only one where sovereignty isn't shared simultaneously, but alternately. For six months a year, Pheasant Island is French; for the other six, it is Spanish.

In the more than three and a half centuries since the treaty, the island has passed back and forth over 700 times between both countries. It is like the ball in an extremely slow game of ping-pong between France and Spain [8]. Or maybe a geographically more apt metaphor is that of the timeshared apartment - an arrangement that would become popular on the Spanish costas from the 1960s onwards.

Now think back to that poor border buff, stuck on the bay path on the French side, or staring out from the Calle Thalamas Labandibar on the Spanish side. As with Tantalus, the object of his desire seems within his grasp [9], but remains unreachable.

I have some travel advice for him: Go north, and find the Luxembourg town of Schengen. That name should ring a bell with him. It's the place where, in 1985, several European countries decided to abolish internal border controls. Today, the Schengen Area covers 26 European countries [10]. Significantly, the Schengen Agreement was signed on the Marie-Astrid, a riverboat moored near the tripoint between Luxembourg, Germany and France in the Moselle River.

The proximity of that tripoint was probably symbolic enough for the signatories. But in actual fact, the place was even more apt for an attempt to efface Europe's national borders. South of Schengen, where the Moselle forms the border between France and Luxembourg, the boundary line runs through the middle of the river [11].

North of the tripoint, the border fans out from a line to a zone. Stretching from bank to bank, it follows almost the entire 84 miles of the German-Luxembourg border north to its end, the tripoint with Belgium. This is the German-Luxembourg condominium, covering the surface of the three rivers that form both countries' common border: the Moselle (Mosel in German), its tributary the Sauer (Sûre in French) and its tributary, the Our [12].

Unlike Pheasant Island, there are no restrictions on public access to this condominium, which includes about 15 river islands of varying size. Perhaps the most interesting one being the apparently nameless one just east of the tripoint with France. The border slices right through its tip. Below, the main mass of land is French. Above, the small sliver is both German and Luxembourgish.

The German-Luxembourg condominium came into being as one of the many borders redrawn by the Vienna Congress in 1815. The parties involved were Prussia, the predecessor state of Germany, and the Netherlands [13]. The Vienna Treaty stipulated that "the rivers themselves, where they form the border, belong in common to the two neighboring Powers" [14].

Condominiums are usually temporary, and continue to exist only by the goodwill of both parties involved. In the 1930s, Nazi Germany evidenced its lack of appetite for international cooperation by proposing to end the condominium. Luxembourg bravely countered that this would require the permission of all parties to the Vienna Treaty.

The curious arrangement was saved, and survives to this day. But not without the problems inherent to joint sovereignty over a river. One such problem being: does joint sovereignty over a river extend to the bridges that cross it?

In 1884, a German court ruled that bridges were not an extension of the river, hence that the border was a median line across the bridge. But this reversal of cujus est solum [15] was undone by the German-Luxembourg Border Treaty of 1984. Which means that if you stand on the bridge between, for example, Echternach in Luxembourg and Echternacherbrücke in Germany, you're in both countries at the same time.

So what if you commit a crime on that bridge now? And how far up the riverbanks does the condominium extend? And what if grand engineering projects alter the course of the river?

None of these questions were answered sytematically by the 1984 border treaty. Which means that Germany and Luxembourg need informal, ad hoc agreements covering every potential area of legal dispute.

Why don't they just cut and run, and chop the river in half as everybody else seems to do? Because a linear division causes its own set of problems. Usually, borders that follow rivers run along the center of the river's main navigable channel, what international law refers to as the thalweg principle. But this makes it very difficult to determine on which side of the border they are at any given time. Shipping channels don't necessarily follow the center of the river, and can shift over time, all without any evidence on the surface.

Thus, even though it is the rarest of border arrangements, a condominium has its advantages. For its rarity stems not from its lack of success, but rather from the continued need for pragmatism over dogmatism.

Denmark and Canada, take note. Both countries are normally known for being boringly sensible, but have repeatedly exchanged harsh words over Hans Island, near Greenland, which is a part of the Kingdom of Denmark. Officials of both countries have visited the island, waving and planting their respective flags.

Hans is a barren rock in Nares Strait, and has the impertinence to lie exactly on the border that separates Canada's Ellesmere Island from Greenland. Since the melting of Arctic ice has opened the region to valuable shipping and resource extraction, neither of the Arctic powers is willing to relinquish the island to the other.

Three options present themselves: a war between Denmark and Canada; a militarized border across the frozen island; or a condominium. The first two are impractical and unlikely. The latter solution would ensure that nothing would happen to the island (or to resource exploration around it) without the permission of both countries. Would this not ensure that the most sensible, and most equitable proposals would prevail?

One final suggestion: When Ottawa and Copenhagen get round to erecting a monument on the island, it would perhaps be best not to use white marble, that high up in the Arctic. A big black monolith, nicely visible from afar, would be the ideal way to commemorate the Danish-Canadian condominium over Hans Island.

Frank Jacobs is a London-based author and blogger. He writes about cartography, but only the interesting bits.

The World's Most Exclusive Condominium

[1] www.hendaye-tourisme.fr.

[2] Also spelled "ait," a small riverine island usually formed by sediment deposits.

[3] Both Basque cities, the former in the northern, French part and the latter in the southern, Spanish part.

[4] The treaty, which stipulated among other things that all villages north of the Pyrenees would be French, created the Spanish exclave of Llivia: north of the mountains, but a town rather than a village.

[5] In Bordeaux and Burgos respectively. Marriages by proxy were not uncommon in the noble and royal circles of medieval Europe.

[6] Île des Faisans in French, Isla de los Faisanes in Spanish, Konpantzia in Basque.

[7] It is also known as Île de l'Hôpital.

[8] Reminiscent of the pace of John Cage's "ASLSP," currently being performed by the organ of the St Burchardi in Halberstadt, Germany, which was started in 2001 and should end in 2640, having lasted 639 years.

[9] At its closest, the distance from Pheasant Island to the riverbank is no more than 100 feet on the Spanish side, about 150 feet on the French side.

[10] All European Union countries except Britain and Ireland, plus Switzerland, Norway and Iceland.

[[11] The condominium is interrupted at Vianden, where Luxembourg extends east of the Our for a stretch three miles.

[12] From 1815 to 1890, a personal union connected the Netherlands to Luxembourg (the Dutch king also being Luxembourg's grand duke). If you're wondering why the Dutch and Luxembourg flags are almost identical, this is it.

[13] It sounds even flowerier in the original French: "[L]es rivières elles-mêmes, en tant qu'elles forment la frontière, appartiendront en commun aux deux Puissances limitrophes." As quoted in Daniel-Erasmus Khan's meticulous 2004 masterpiece "Die deutschen Staatsgrenzen: rechtshistorische Grundlagen und offene Rechtsfragen" ("The German National Border: Principles of Legal History and Open Legal Questions").

[14] "Cuius est solum, eius est usque ad coelum et ad inferos": "Who owns the land, owns it all the way up to heaven, and all the way down to hell." No longer considered practical since, well, the advent of the hot air balloon.