The earliest known version of the Tristan and Isolde story, on which some later medieval versions are based, was written around 1150, although this version of the story no longer exists. This version drew on oral legends of the story that stretched back to an indiscernible period in Celtic antiquity and were popular in the regions that would become modern-day Britain, France, and Germany. Tristan and Isolde were legendary characters, similar in this respect to other figures who attained a legendary status in oral legends and literary works, such as King Arthur or the Germanic heroes Siegfried and Dietrich. In fact, the French prose work Tristan, written in the 13th century, would integrate the story of Tristan and Isolde into Arthurian legend, making Tristan a Knight of the Round Table. The shape of the story would also significantly influence the Arthur-Guinevere-Lancelot love triangle of Arthurian legend, in which the knight’s duty to his lord conflicts with his duty to his lady.

Despite the many permutations of the story, certain elements tended to remain constant: Tristan and Mark’s familial relationship, Tristan’s entrustment with delivering Isolde to Mark, and Tristan and Isolde’s consumption of a love potion that sparks their romantic entanglement.