Cornerstone
A cornerstone was a  or brick that was placed in the corner of a structure.

The cornerstone of every building in the Caldos colony came from various places in Scotland, including the cities of Edinburgh, Glasgow, and Aberdeen. Maturin explained to Jean-Luc Picard in 2370 that it was done by the colony's founders so that the colonists could "feel they had a piece of the real thing here". ()

Metaphorically, a cornerstone referred to anything that provided a defining or stabilizing influence. (, , )

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Architecture