We don't have synonyms for cotherstone yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are cornerstone, copperstone and atherstone.
°A ceremonial stone set at the corner of a building, joining two exterior walls, and often inscribed with the starting and completion dates of construction, the name of the architect and owner, and other details.
"The cornerstone on the Flatiron Building is set on the Fifth Avenue facade."
°A ceremonial stone set at the corner of a building, joining two exterior walls, and often inscribed with the starting and completion dates of construction, the name of the architect and owner, and other details.
"The cornerstone on the Flatiron Building is set on the Fifth Avenue facade."