A star system containing the warring planets Elas and Troyius, lying close to Klingon territory and claimed by the Klingon Empire in Elaan of Troyius.
The Tellun system is the setting of the Original Series episode Elaan of Troyius. Its two inhabited planets, Elas and Troyius, have developed space travel and with it the means to destroy each other, and their rulers have agreed to end the feud by marrying Elaan, the Dohlman of Elas, to the leader of Troyius.
The Federation supports the peace because the system sits near the Klingon border, and the Klingons claim authority over it. The Enterprise carries Elaan between the two planets on impulse power while the Troyian ambassador Petri tries to teach her his people's manners.
A Klingon warship shadows the voyage and attacks once sabotage has disabled the warp drive. The reason becomes clear when Elaan's necklace turns out to be made of dilithium crystals, which are common stones on Elas.