The “Treaty of Treaties”→

Wikipedia:

The Convention codifies several bedrocks of contemporary international law. It defines a treaty as “an international agreement concluded between states in written form and governed by international law,” as well as affirming that “every state possesses the capacity to conclude treaties.” The most important point in the Convention is that Article 1 restricts the application of the Convention to written treaties between States, excluding treaties concluded between the states and international organizations or international organizations themselves.
Most nations, whether they are party to it or not, recognize it as the preeminent “Treaty of Treaties”

Sometimes getting lost in Wikipedia is a lot of fun.