The First Contract signed by both sides of the Border.
It is first referred to in Unto Zeor, Forever. The events leading up to it and the social climate surrounding the First Contract, which ushers in Unity, are recounted in Zelerod's Doom, To Kiss or to Kill, and The Story Untold
That first treaty between Territories is of course renegotiated and later reformed many times as Territories from around the world join the Modern Tecton, eventually forming the World Tecton.
(Source: Index card file, Unto Zeor, Forever) First Contract between the Territories. The Tecton was responsible for controlling asocial behavior of all Simes --- Controller's injunctions to help bring this about (with penalty of death by Attrition.) The First Contract was signed by Klyd Farris.