Ceremonial building block, dated or otherwise inscribed, usually placed in an outer wall of a building to commemorate its dedication. Often the stone is hollowed out to contain newspapers, photographs, or other documents reflecting current customs, with a view to their historical use when the building is remodeled or demolished. Originally placed at a corner, the stone may today be placed elsewhere on the facade.
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The cornerstone (or foundation stone) concept is derived from the first stone set in the construction of a masonry foundation, important since all other stones will be set in reference to this stone, thus determining the position of the entire structure.
Over time a cornerstone became a ceremonial masonry stone, or replica, set in a prominent location on the outside of a building, with an inscription on the stone indicating the construction dates of the building and the names of architect, builder and other significant individuals. The rite of laying a cornerstone is an important cultural component of western architecture and metaphorically in sacred architecture generally.
Some cornerstones include time capsules from the time a particular building was built. The origins of this tradition are vague but its presence in Judeo-Christian countries can be associated with one quotation from the Old Testament cited six times in the New Testament (,, , , and ).
Often, the ceremony involved the placing of offerings of grain, wine and oil on or under the stone. These were symbolic of the produce and the people of the land and the means of their subsistence. This in turn derived from the practice in still more ancient times of making an animal or human sacrifice that was laid in the foundations.
Nowhere, perhaps, does the equivalence of the shadow to the life or soul come out more clearly than in some customs practised to this day in South-eastern Europe. In modern Greece, when the foundation of a new building is being laid, it is the custom to kill a cock, a ram, or a lamb, and to let its blood flow on the foundation-stone, under which the animal is afterwards buried. The object of the sacrifice is to give strength and stability to the building. But sometimes, instead of killing an animal, the builder entices a man to the foundation-stone, secretly measures his body, or a part of it, or his shadow, and buries the measure under the foundation-stone; or he lays the foundation-stone upon the man's shadow. It is believed that the man will die within the year. The Roumanians of Transylvania think that he whose shadow is thus immured will die within forty days; so persons passing by a building which is in course of erection may hear a warning cry, Beware lest they take thy shadow! Not long ago there were still shadow-traders whose business it was to provide architects with the shadows necessary for securing their walls. In these cases the measure of the shadow is looked on as equivalent to the shadow itself, and to bury it is to bury the life or soul of the man, who, deprived of it, must die. Thus the custom is a substitute for the old practice of immuring a living person in the walls, or crushing him under the foundation-stone of a new building, in order to give strength and durability to the structure, or more definitely in order that the angry ghost may haunt the place and guard it against the intrusion of enemies.
Often still, and certainly until the 1970s, most ceremonies involved the use of a specially manufactured and engraved trowel that had a formal use in laying mortar under the stone. Similarly, a special hammer was often used to ceremonially tap the stone into place.
The foundation stone often has a cavity into which is placed a time capsule containing newspapers of the day or week of the ceremony plus other artifacts that are typical of the period of the construction: Coins of the year may also be immured in the cavity or time capsule.
The establishment of a new Masonic building is accompanied by a ceremony that perpetuates the formal ceremonies described above.
A cornerstone (Greek: Άκρογωνιεîς, Latin: Primarii Lapidis) will sometimes be referred to as a "foundation-stone", and is symbolic of Christ, whom the Apostle Paul referred to as the "head of the corner" and is the "Chief Cornerstone of the Church" (). Many of the more ancient churches will place relics of the saints, especially martyrs, in the foundation stone.
After this, the Litany of the Saints is said, followed by an antiphon and (Psalm 126 in the Septuagint), which appropriately begins with the verse, "Except the Lord build the house, in vain do they labour that build it". Then the stone is lowered into its place with another prayer and again sprinkled with holy water. More antiphons and psalms follow, while the bishop sprinkles the foundations, dividing them into three sections and ending each with a special prayer. Finally, Veni Creator Spiritus is sung, and two short prayers. Then the bishop, if he deems it opportune, sits down and exhorts the people to contribute to the construction, appointments and maintenance of the new church, after which he dismisses them with his blessing and the proclamation of an indulgence.