A reclusive non-humanoid species obsessed with legal precision, whose treaty with the Federation forced the evacuation of the human colony on Tau Cygni V.
The Sheliak are a reclusive, non-humanoid species who regard most humanoids as an inferior form of life and communicate only rarely with the Federation. Their relationship with it is governed by the Treaty of Armens, signed in 2255, a vast and minutely detailed document that the Sheliak insisted upon because they regard ordinary language as imprecise.
The treaty cedes a number of worlds to the Sheliak, including Tau Cygni V. In 2366, in the episode The Ensigns of Command, the Sheliak discover that a human colony has settled there without permission and demand its removal within days, threatening to exterminate the colonists otherwise. Data is sent to persuade the stubborn settlers to leave, while Jean-Luc Picard searches the treaty for room to manoeuvre.
Picard finally exploits a clause allowing either side to call for third-party arbitration, and since the chosen arbiters are unavailable for months, the Sheliak agree to grant the colonists three weeks instead. The episode is remembered as a lesson in turning an opponent's love of rules against it.