Langdon could scarcely believe his own supposition, and yet, considering who had given this stonecylinder to them, how he had given it to them, and now, the inlaid Rose on the container, Langdoncould formulate only one conclusion.

　　I am holding the Priory keystone.

　　The legend was specific.

　　The keystone is an encoded stone that lies beneath the sign of the Rose.

　　"Robert?" Sophie was watching him. "What's going on?"Langdon needed a moment to gather his thoughts. "Did your grandfather ever speak to you ofsomething called la clef de vo.te?""The key to the vault?" Sophie translated.

　　"No, that's the literal translation. Clef de vo.te is a common architectural term. Vo.te refers not to abank vault, but to a vault in an archway. Like a vaulted ceiling.""But vaulted ceilings don't have keys.""Actually they do. Every stone archway requires a central, wedge-shaped stone at the top whichlocks the pieces together and carries all the weight. This stone is, in an architectural sense, the keyto the vault. In English we call it a keystone." Langdon watched her eyes for any spark ofrecognition.

　　Sophie shrugged, glancing down at the cryptex. "But this obviously is not a keystone."Langdon didn't know where to begin. Keystones as a masonry technique for building stonearchways had been one of the best-kept secrets of the early Masonic brotherhood. The Royal ArchDegree. Architecture. Keystones. It was all interconnected. The secret knowledge of how to use awedged keystone to build a vaulted archway was part of the wisdom that had made the Masonssuch wealthy craftsmen, and it was a secret they guarded carefully. Keystones had always had atradition of secrecy. And yet, the stone cylinder in the rosewood box was obviously somethingquite different. The Priory keystone—if this was indeed what they were holding—was not at allwhat Langdon had imagined.

　　"The Priory keystone is not my specialty," Langdon admitted. "My interest in the Holy Grail isprimarily symbologic, so I tend to ignore the plethora of lore regarding how to actually find it."Sophie's eyebrows arched. "Find the Holy Grail?"Langdon gave an uneasy nod, speaking his next words carefully. "Sophie, according to Priory lore,the keystone is an encoded map... a map that reveals the hiding place of the Holy Grail."Sophie's face went blank. "And you think this is it?"Langdon didn't know what to say. Even to him it sounded unbelievable, and yet the keystone wasthe only logical conclusion he could muster. An encrypted stone, hidden beneath the sign of theRose.

　　The idea that the cryptex had been designed by Leonardo da Vinci—former Grand Master of thePriory of Sion—shone as another tantalizing indicator that this was indeed the Priory keystone. Aformer Grand Master's blueprint... brought to life centuries later by another Priory member. Thebond was too palpable to dismiss.

　　For the last decade, historians had been searching for the keystone in French churches. Grailseekers, familiar with the Priory's history of cryptic double-talk, had concluded la clef de vo.te wasa literal keystone—an architectural wedge—an engraved, encrypted stone, inserted into a vaultedarchway in a church. Beneath the sign of the Rose. In architecture, there was no shortage of roses.

　　Rose windows. Rosette reliefs. And, of course, an abundance of cinquefoils—the five-petaleddecorative flowers often found at the top of archways, directly over the keystone. The hiding placeseemed diabolically simple. The map to the Holy Grail was incorporated high in an archway ofsome forgotten church, mocking the blind churchgoers who wandered beneath it.

　　"This cryptex can't be the keystone," Sophie argued. "It's not old enough. I'm certain mygrandfather made this. It can't be part of any ancient Grail legend.""Actually," Langdon replied, feeling a tingle of excitement ripple through him, "the keystone isbelieved to have been created by the Priory sometime in the past couple of decades."Sophie's eyes flashed disbelief. "But if this cryptex reveals the hiding place of the Holy Grail, whywould my grandfather give it to me? I have no idea how to open it or what to do with it. I don'teven know what the Holy Grail is!"Langdon realized to his surprise that she was right. He had not yet had a chance to explain toSophie the true nature of the Holy Grail. That story would have to wait. At the moment, they werefocused on the keystone.

　　If that is indeed what this is....

　　Against the hum of the bulletproof wheels beneath them, Langdon quickly explained to Sophieeverything he had heard about the keystone. Allegedly, for centuries, the Priory's biggestsecret—the location of the Holy Grail—was never written down. For security's sake, it wasverbally transferred to each new rising sénéchal at a clandestine ceremony. However, at some pointduring the last century, whisperings began to surface that the Priory policy had changed. Perhaps itwas on account of new ele............