The stone placed in the centre of an arch which preserves the others in their places, and secures the firmness and stability to the arch. As it was formerly the custom of Operative Masons to place a peculiar mark on each stone of the building to designate the workman by whom it had been adjusted, so the Keystone was most likely to receive the most prominent mark, that of the superintendent of the structure. Such is related to have occurred to that Keystone which plays so important a part in the legend of the Royal Arch Degree. The objection has sometimes been made, that the arch was unknown in the time of Solomon. But this objection has been completely laid to rest by the researches of antiquaries and travellers within a few years past. Wilkinson discovered arches with regular keystones in the doorways of the tombs of Thebes, the construction of which he traced to the year 15540 B.C., or 460 years before the building of the Temple of Solomon. And Dr. Clark asserts that the Cyclopean gallery of Tiryns exhibits lancet-shaped arches almost as old as the time of Abraham. In fact, in the Solomonic era, the construction of the arch must have been known to the Dionysian Artificers, of whom, it is a freely received theory, many were present at the building of the Temple. Another shape used is a double keystone; that is, a keystone on top of a larger keystone.