Gottfried states that the Tristan of Thomas of Britain, an Anglo-French work of around 1160, was the source of his work. He explains that he bases himself on Thomas because he "told the tale correctly", distancing himself from the less courtly versions of the story represented by Béroul in Old French and Eilhart von Oberge in Middle High German.
Unfortunately, Thomas's work, too, is fragmentary and there is little overlap with Gottfried's poem, making it difficult to evaluate Gottfried's originality directly. However, Thomas's Tristan was the source of a number of other versions, which makes it possible to get some idea of style and content. It is clear that while Gottfried's statement of his reliance on and debt to Thomas is correct, he both expanded on his source and refined the story psychologically. The discovery in 1995 of the Carlisle Fragment of Thomas's Tristan, which includes material from one of the central parts of the story, the Love Grotto episode, promises a better understanding of Gottfried's use of his source.
Thomas's source, in turn, is a now lost Old French Tristan story, reconstructed by Joseph Bédier, which derives ultimately from Celtic legend.
