Neutral Zone Treaty

From Memory Alpha, the free Star Trek reference

The Neutral Zone Treaty specified legislation in the Neutral Zone.

In 2287, Spock informed Sybok that he was under arrest for committing seventeen violations of the Neutral Zone Treaty, following Sybok's capture of Paradise City and victory over the Starfleet rescue party. (Star Trek V: The Final Frontier)

The creation of the experimental joint Klingon, Federation, Romulan colony on the planet Nimbus III, located in the Neutral Zone in 2267, would suggest the treaty dates back to that time. It is also possible this is the same treaty described in TOS: "Balance of Terror" signed after the end of the Earth-Romulan War.