The origin of the Tristan myth is lost in antiquity. The Welsh Triads,
of unknown date, but very ancient, know of one Drystan ab Tallwch, the
lover of Essylt the wife of March, as a steadfast lover and a mighty
swineherd. It is indubitably Celtic-Breton, Irish, or Welsh. There
were different versions of the story, into the shadowy history of
which we need not enter; the only one which concerns us is that of a
certain "Thomas." Of his French poem fragments alone have come down to
us, but we have three different versions based upon it:

1. The Middle-High-German poem of Gottfried von Strassburg, composed
about 1210-20. 2. An old-Norse translation made in 1226 by command of
King Hakon. 3. A Middle-English poem of the thirteenth century
preserved in the so-called Auchinleck MS. of the library of the
Faculty of Advocates in Edinburgh, and familiar to English readers
from the edition published by Sir Walter Scott. The poem was probably
composed by the famous Thomas the Rhymer of Ercyldoune or Earlstown in
Berwickshire. A reliable edition by G. P. MacNeill has been published
by the Scottish Text Society, with an introduction giving a full and
interesting account of the legend in its various recensions.

In these versions the story of Tristan and Isolde has nothing whatever
to do with the Arthurian court or the quest of the Grail. It became
exceedingly popular and was told again and again in varied forms in
every language in Europe. But even before this Sir Tristan had
sometimes been included among the Knights of the Round Table, such
honour being deemed indispensable to the dignity of every knight who
had any pretensions to fame.