keystone
UK[ˈkiːstəʊn]US[ˈkiːstoʊn]
n
the central stone at the top of an arch that locks all the other stones in place.
n
the most important part of a plan, idea, etc., on which everything else depends.
Morpheme Breakdown
key
stone
key
stone
Etymology
the word 'keystone' is a transparent compound formed from the native english elements 'key' and 'stone'. it originated in the early 17th century as a direct architectural term, drawing a metaphorical link between the function of a physical key that locks a mechanism and the central stone that locks an arch's structure together. this literal meaning was then extended figuratively by the mid-18th century to denote the central, indispensable element of any system or idea, mirroring the structural indispensability of the architectural component. the logic of the word's formation remains immediately apparent, with the modern abstract sense flowing directly from the concrete, functional analogy of its parts.
Analysis
Structure: key + stone
key: from middle english 'keye', old english 'cǣg', meaning a device for locking/unlocking or a crucial part.
stone: from old english 'stān', meaning a piece of rock.
Examples
the keystone of the bridge was carefully set in place by the master mason.
trust is the keystone of any successful relationship.
the new evidence became the keystone of the prosecutor's argument.