The Federation Neutral Zone in 2291.

The Federation–Klingon Neutral Zone, also known as the Organian Neutral Zone and the Organian Treaty Zone, was a region of space that served as a neutral boundary between the United Federation of Planets and the Klingon Empire during the 23rd century.

In 2251, the USS Republic, under the command of Captain Rollin Bannock, was dispatched on routine training mission near the Neutral Zone. One night, the Republic was assigned a classified mission which meant that all cadets aboard, including James T. Kirk and Gary Mitchell, were confined to crew quarters for the duration. (TOS - My Brother's Keeper novels: Republic, Constitution)

Around seven years later, Starfleet Command ordered Captain Augenthaler to take the USS Constitution on a classified mission to an unnamed star system located a few light-years from the Neutral Zone, to a location where the Republic had also been dispatched to several years earlier. (TOS - My Brother's Keeper novel: Constitution)

These references to a Klingon Neutral Zone prior to the Organian Peace Treaty likely refer to an earlier Federation–Klingon border, which was likely redrawn with the Treaty.

The neutral boundary was negotiated by representatives of both powers over a period of three months following the imposition of the Organian Peace Treaty by the Organians in 2267. As a result of the negotiation, both sides gained additional territory in some respect. The new treaty discussion provided clauses for both powers to develop planets within the Zone. However, the disposition of planets residing within the zone remained unresolved at the time, until there were efforts made by either side to positively utilize resources therein. (FASA RPG module: Star Trek IV Sourcebook Update)

The Orions, lying between both powers, were ignored and excluded by the Organians in their considerations. However, the Orion Neutrality Area remained intact, overlapping the Neutral Zone and bordering both powers. This area was not under Organian jurisdiction, which enabled both Orion and Klingon ships to sneak over the border into Federation space. The Orions were the only power to really benefit from the Organian Peace Treaty, gaining the Neutral Zone to explore and operate in relative freedom. (FASA RPG module: The Orions: Book of Common Knowledge)

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