The island that switches countries every six months

Next week, France will hand over 3,000 sq m (32,000 sq ft) of its territory to Spain without a single shot being fired. But in six months' time Spain will voluntarily hand back the land to France. It's been that way for more than 350 years.

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The Island of Faisans (Pheasant Island)

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But there it is - a peaceful, inaccessible island in the middle of the river, with tree cover and neatly trimmed grass, and an old monument which pays tribute to a remarkable historical event that happened here in 1659.
For three months, the Spanish and French negotiated the end to their long war on the island, as it was considered neutral territory. Wooden bridges were extended from both sides. The armies stood ready as the negotiations began.
A peace agreement was signed - the Treaty of the Pyrenees. Territory was swapped and the border demarcated. And the deal was sealed with a royal wedding, as the French King Louis XIV married the daughter of the Spanish King Philippe IV.

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http://www.bbc.com/news/stories-42817859

These are called Condominium Properties or Condominia
The River Moselle and the Sauer and the Orr Tributaries are condominium between Luxembourg and Germany
Gule of Fonseca is a Tridominium between Honduras, El Salvador and Nicaragua
Parts of the Parana River and the mouth of the Iguassu River are shared between Brazil and Paraguay

Some former condominia include
688 AD Cyprus between the Byzantine Empire (Justinian II) and the Arab Empire ( Caliph Abd al-Malik ibn Marwan)
1941-3 Croatia was a condominium between Germany and Italy
1876-82 Egypt was one under Britain and France
1992-94 Walvis Bay Namibia and South Africa
Schleswig, Holstein and Lauenburg were an Austrian-Prussian condominium
The Oregon Country was an Anglo-American condominium from 1818 until 1846
A small area (Hadf and surroundings) on the Arabian Peninsula, a part of Oman, at one time was jointly ruled with the Emirati member state of Ajman.
New Hebrides formed a French–British condominium in 1906 until independence in 1980 as a republic, now called Vanuatu