Officer Exchange Program
From Memory Alpha, the free Star Trek reference.
The Officer Exchange Program was established in the 2360s as a way of improving relations between the United Federation of Planets and the Klingon Empire. Commander William Riker of the USS Enterprise-D volunteered for the program and was posted aboard the IKS Pagh, a Klingon Bird-of-Prey. The Benzite, Mendon, was posted aboard the Enterprise-D as part of the same program. (TNG: "A Matter Of Honor")
Kurn, the second son of Mogh, later joined the program as a way of meeting his brother Worf. (TNG: "Sins of the Father")
The Klingon spy, J'Dan later used the program to give information on the ship to the Romulans. (TNG: "The Drumhead")
