A condominium is a concept in international law that describes a geographic area shared in equal sovereignty by two nations. As a practical matter, it creates a genuinely unusual and often impractical solution. The condominium isn’t distinctly part of any one nation but by agreement it’s within the control of both. It has no standing on its own and has no independent sovereignty. This type of situation does not generally last for long since it depends upon a shared arrangement between independent sovereign parties. Eventually the arrangement collapses as one or the other tires of the it.
Here are some condominiums that exist in the modern world, courtesy of Wikipedia’s condominium page.
SOURCE: Community map from the Commune de Schengen (Luxembourg)
Generally a river border belongs to one country or the other, will run down the thalweg or be otherwise split between them. That is not the case with the Mosel River that runs between Luxembourg and Germany. It is controlled jointly in a condominium arrangement. Notice the circle drawn on the topographic map where France joins the condominium. The French border runs down the middle, but follow it north and notice that the borders of Luxembourg and German BOTH hug their respective banks! The point on the little river island where France and the condominium intersect is often described as the "DEFRLU Tripoint.” I’m not sure if it’s really a tripoint that’s actually a line running along the southern edge of the condominium, or not really a tripoint at all. Either way, it’s very odd.
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There is a second condominium in Europe, a small island on the River Bidasoa between France and Spain: Pheasant Island. Each country maintains control for six months of the year and then turns control over to the other. Historically, this was the site of the 1659 Treaty of the Pyrenees which ended the 30 Years’ War between the two countries. So it’s not simply some obscure plot of land in the middle of a river.
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The final condominium involves a small tip of land on the Arabian Peninsula shared between Oman and the emirate of Ajman (part of the United Arab Emirates). Drill down on this satellite map and you’ll notice what appears to be a small town. This must make for an interesting situation for the inhabitants. Are they citizens of Oman or the UAE? Both? Neither? Are they citizens of anywhere?
One more area that is often considered a condominium is not: Andorra. This nation is often lumped into the condominium category but it’s actually a co-principality and a sovereign state.