In Christian tradition, the Holy Grail is the cup that Jesus drank from at the Last Supper and in which Joseph of Arimathea caught his blood at the crucifixion. The Joseph connection comes along late in the tradition, but it has a venerable history.
The Holy Grail ended up in Britain. Not everything in Monty Python and the Holy Grail is true, but at least according to legend when the Grail disappeared it was taken to England by Joseph of Arimathea. To Camelot (not to confuse a musical with a movie).
Scotland's Rosslyn Chapel, one of the final destinations in the novel, has a code carved into its masonry that has never been cracked. And it's not for lack of trying.
Rosslyn is also an exact replica of the Temple of Solomon. Well, that's stretching the truth. But at least that's the legend.
The rose (a symbol in Dan Brown's book for the Grail) has a long association with secrecy. Brown's allusion to the phrase sub rosa is correct; the Romans did use a rose to symbolize that a meeting was private.
The Holy Grail isn't a cup at allit's an inverted V, the ancient symbol for womanhood. Hard to swallow. In the words of Diane Apostolos-Cappadona, a religious art professor at Georgetown University, "that's a very Jungian reading of Mary Magdalene."
The French phrase for "Holy Grail" derives from the words for "royal blood." The relevant word would be sang, but the more likely derivation is from saint followed by an old word for dish. The Catholic Encyclopedia thinks the "blood" explanation was too fanciful for later writers to resist.
Leonardo left out the central cup of wine in The Last Supper as an allegorical reference to the Grail. It's true there's no central cup of wine in front of the figure of Jesus but it's hard to read much into it.
The 90-degree angle between Jesus and "Mary Magdalene" in the same painting represents the symbol of the chalice (and of the sacred feminine). Actually, it was Leonardo's mastery of perspective and composition that made his work eternal. Everything about the arrangement of the figures in the painting suggests artistic deliberation, not hidden code.
The Grail legends were stories of forbidden quests to find the "sacred feminine." That would have come as news to Geoffrey of Monmouth, on whose 12th-century chronicle of King Arthur and Merlin the Grail story is based.