Other forms: cornerstones
A cornerstone is the first stone laid, and anything a thing depends on. Lay the cornerstone, the cornerstone of the policy, a cornerstone of the argument.
The literal one sets the position of two walls at once, so every other stone follows from it, which is exactly why the figure works. The figurative sense is much the commoner now, and comes partly from the psalm about the stone the builders rejected.
1nouncornerstoneThe first stone laid, and anything a thing depends on.
“Trust is the cornerstone of any good marriage.”
cornerstone is a stone, or a foundation.
Corner plus stone, in Middle English. The figurative use follows Psalm 118 and its echoes in the New Testament.
Middle English
corner ston
English, today
cornerstone
From Middle English corner ston; equivalent to corner + stone. The figurative sense is in part after Psalm 118:22, and is also quoted several times in the New Testament.
Etymology from Wiktionary, CC BY-SA