A cornerstone is the first stone set in the construction of a masonry foundation. In operative masonry, all other stones are laid in reference to this one, making it a symbol of strength, stability, and proper alignment. In speculative Freemasonry, the cornerstone represents moral and spiritual foundations — the principles on which a Mason builds his life.

When a Masonic cornerstone ceremony is held, a symbolic stone (often engraved with the date, names of officials, or Masonic emblems) is prepared and ceremonially “laid” using the traditional working tools of the Craft: the square, level, and plumb. The stone is declared to be “plumb, square, and level”, representing integrity, uprightness, and truth.