Tripoint

A tripoint, trijunction, triple point, or tri-border area is a geographical point at which the boundaries of three countries or subnational entities meet. There are approximately 176 international tripoints. Nearly half are situated in rivers, lakes or seas. On dry land, the exact tripoints may be indicated by markers or pillars, and occasionally by larger monuments.
Usually, the more neighbours a country has, the more international tripoints that country has. China with 16 tripoints and Russia with 11 to 14 lead the list of states by number of tripoints. Within Europe, landlocked Austria has nine tripoints, among them two with Switzerland and Liechtenstein. Island countries such as Japan have no country tripoints (some, like Bahrain and Singapore, have tripoints in the territorial waters), and the same goes for states with only one neighbour state, like Portugal or Lesotho. Likewise, the United States with two neighbour states has no country tripoints; it has a number of tristate points as well as one point where four states meet. Canada, as well, has five tripoints on land where the boundaries of provinces and territories meet, including one quadripoint where four provinces and territories meet.
Border junctions (or "multiple points" or "multipoints" as they are also sometimes called) are most commonly threefold. There are also a number of quadripoints, and a handful of fivefold points, as well as probably unique examples of a sixfold, sevenfold, and eightfold point. No more than eight borders meet at a single multipoint anywhere on earth, but the territorial claims of six countries converge at the south pole in a point of elevenfold complexity.
Examples.
International tripoints include:
Some historic tripoints:
For a full list, see list of tripoints.
International agreements.
While the exact line of an international border is normally fixed by a bilateral treaty, the position of the tripoints may need to be settled by a trilateral agreement. For example, China, Russia, and Mongolia have set the position of the two relevant tripoints (the junction points of the China–Russia border, the Mongolia–Russia border, and the China–Mongolia border) by the trilateral agreement signed in Ulaanbaatar on January 27, 1994. The agreement specified that a marker was to be erected at the eastern tripoint, called Tarbagan-Dakh, but that no marker would be erected at the western tripoint (which was defined as the peak of the mountain Tavan-Bogdo-Ula (Kuitunshan, Tavan Bogd Uul).