Cornerstone

A stone placed at the angle, or corner, of a building where two walls meet, important in joining and binding them together. The principal cornerstone was the foundation cornerstone; a particularly strong one was generally chosen for public buildings and city walls. The word is used in a figurative sense for the founding of the earth, and Jesus is spoken of as “the foundation cornerstone” of the Christian congregation, which is likened to a spiritual house.​—Eph 2:20; Job 38:6.