The seat was born of a pattern, and the pattern is holy. Robert de Boron, writing around 1200, taught that there had been three tables in the history of grace: the table of the Last Supper, where one place stood empty for Judas the traitor; the table that Joseph of Arimathea made for the Holy Grail, where the empty seat of Judas waited for a worthier sitter; and Arthur's Round Table, the third and last, which Merlin made for Uther Pendragon in their image. The Round Table could not be a copy of those tables without their empty place, and so Merlin ordained that one siege of the fellowship remain unfilled, kept against a knight who was promised and had not yet come. The fellowship of the greatest knights in the world was thus founded with a hole in it, a visible absence at the heart of the feast, and the seat took its name from its nature: in the French of the romances a siege is simply a seat, and this one was perilous.

It earned the name. In the Grail history of the Vulgate Cycle, a false disciple named Moyses presumed to sit in the empty seat of Joseph's table, though he had been warned, and the earth opened beneath him and took him alive, and his voice cried up out of the ground as a warning to every later generation. The romances remember others who dared and were destroyed, and the seat's deadliness became part of the furniture of Camelot, as fixed as the table itself. Nor was the seat merely fatal; it was articulate. Letters of gold appeared upon it by enchantment and foretold its own fulfilment, and Malory preserves the marvellous detail: the writing declared that four hundred winters and four and fifty, accomplished after the Passion of Our Lord, the siege should be filled. For years the knights sat down to dine beside a countdown.

The filling of it is one of the great scenes of the legend. On the feast of Pentecost, Malory tells, the court at Camelot saw first a marvel on the river: a great stone of red marble floating against the stream, with a sword stuck through it, and letters promising that only he should draw it by whose side it ought to hang, the best knight of the world. Then a good old man led into the hall a young knight in red arms, carrying neither shield nor sword, and brought him to the Siege Perilous, where the company found the letters newly changed: this is the siege of Galahad, the haut prince. Lancelot reckoned the years and found the day had come. The young knight sat, and no harm came to him, and the whole court wondered, and that evening the Holy Grail entered the hall covered in white samite and fed every knight with the food he most desired, and vanished, and the quest was sworn. Arthur alone did not rejoice. He knew that the empty seat had held the fellowship together like a keystone, and that the man who filled it had come to empty the hall.

That is the seat's meaning, and it is as sharp now as it was in the year 1200. Every fellowship that ranks itself by merit keeps, whether it admits it or not, a siege perilous: a place at the table that prowess cannot claim, reserved for a standard beyond strength and beyond lineage, for grace, for purity, for the thing the company cannot supply itself. The Round Table declared all its knights equal, and the Siege Perilous declared that equality insufficient. It was the conscience of Camelot made visible, the Grail's ambassador seated among the swords, and its lesson is the legend's saddest: the day the seat was filled was the day the breaking began. The phrase has wandered far from the table since - modern fantasy has borrowed it for a jewel that judges souls, and T.H. White kept it standing in his own doomed Camelot - but it has never lost its first sense. Some chairs are not for sitting. Some are for measuring everyone else.