The word grail is generally used nowadays to mean something unknown but supremely important and desirable, which, if it can be discovered, will transform the world in which the grail exists. So, for instance, it could be said that the Holy Grail of twentieth-century physics was a ‘Theory of Everything’. This use of the word grail is a metaphorical development of stories that grew up in the Middle Ages within the Arthurian legend. The metaphorical sense fits all of them, but they have little else in common. As R.S. Loomis said, they seem to delight in contradicting each other on important points: on the number of kings in the Grail castle, their names, their physical state, the cause and exact nature of ‘the Waste Land’, the name of the successful hero, and the form and attributes of the Grail itself. We might add to Loomis's list that they also disagree about the objects and ceremonies associated with the Grail, where it is to be found, what the hero has to do to ‘achieve’ it and the nature of the transformation that the achievement will bring about.

Their treatment of the Grail itself illustrates that range of variation. Richard O'Gorman, in an essay in the standard reference work, listed the forms that the grail takes in the principal texts. His list presents some problems, but they do not affect the present argument. He notes that the earliest surviving Grail romance, Chrétien de Troyes's Perceval, introduces the central object not as the Grail but as a grail, which suggests that it was not unique but a member of a class of objects that Chrétien's audience would be able to visualize.