The Klingon-Romulan alliance was an alliance formed in the 23rd century between the Klingon Empire and Romulan Star Empire.
One of the aims of the alliance, spearheaded by the likes of Proconsul Merrok was for the two empires to work together to contain the Cardassian Union. (DS9 novel: A Stitch in Time)
Another outcome of the alliance was a technology exchange between the two empires, the Romulans gained advanced D7-class battle cruisers, whilst the Klingons were given cloaking devices (TOS episode: "The Enterprise Incident"; TOS movie: Star Trek III: The Search for Spock)
The next generation Klingon Bird-of-Prey, the B'rel-class incorporated Romulan designs, including of course, a cloaking device. (ST reference: Starship Spotter)