The formula “the islands of the sea” occurs three times in Le Conte del Graal, by Chrétien de Troyes. Two of the instances refer to the homeland of Perceval's parents, and critics have assumed that the Hebrides are meant. But this sense is quite unsuitable for the third instance. Although the formula could have general application to islands, it is significant that in connection with Arthur it appeared, as “the Islands of the Ocean”, in the Historia Regum Britanniae by Geoffrey of Monmouth, in passages that reflect the ancient Welsh tradition that the whole realm of a ruler of Britain was the island of Britain along with its adjacent islands. In Le Conte del Graal the formula only occurs in expressions of incomparability, and in other Arthurian romances there are several instances of its employment in expressions of a similar rhetorical nature. The tradition that the whole realm of Britain had consisted of the island of Britain along with its adjacent islands would best explain the employment of the formula “the islands of the sea” in such cases.