Lord Petri was a Troyian ambassador to the Elasians, in 2268.

In spite of his antipathy for the people of Elas, Petri hoped the long war between the two worlds of the Tellun system could be brought to an end before their mutual destruction. An arranged marriage of the royal Elasian Dohlman Elaan to the ruler of Troyius brought a chance for peace. Fearing the arrogant and demanding Dohlman might prove unacceptable to the Troyian people, the Elasian Council of Nobles and the Troyius Tribunal jointly assigned Petri to Elaan's escort, in order to "make her acquainted" with the relatively genteel Troyian customs.

In transit to Troyius aboard the USS Enterprise, imperious Elaan and condescending Petri were ill-suited for a cultural exchange. After arguments and tantrums, Elaan stabbed Petri in the back. After refusing further dealings with the Dohlman, Captain James T. Kirk assumed the role of Elaan's instructor in basic polite manners. (TOS: "Elaan of Troyius")

Petri was played by actor Jay Robinson.
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