keystone

American  

[kee-stohn] / ˈkiˌstoʊn /

noun

1. the wedge-shaped piece at the summit of an arch, regarded as holding the other pieces in place.

2. something on which associated things depend.

   the keystone of one's philosophy.

   Synonyms:

   linchpin, foundation, principle, basis

3. Also called keystone sack.  Baseball Slang. second base.

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keystone British  

/ ˈkiːˌstəʊn /

noun

1. Also called: headstone.   quoin.  the central stone at the top of an arch or the top stone of a dome or vault

2. something that is necessary to connect or support a number of other related things

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Etymology

Origin of keystone

First recorded in 1630–40; key ¹ + stone

Explanation

In a curved stone archway, the keystone is the one at the very center of the top. The keystone is the most important stone, and that's why this word is also used figuratively to mean the most important part of anything. A stone arch or vault gains its stability from the placement of the keystone, which is often the last one placed. From this architectural meaning, keystone also gained the figurative meaning of "central or most important part," or "the one thing upon which everything else depends." Like an actual stone keystone, this type lends stability: "The right to free speech is the keystone of all American rights."

Example Sentences

She pointed to the 1967 Outer Space Treaty, a keystone document on international space law that President Lyndon Johnson signed onto.

From Salon • Apr. 9, 2026

Morgan healthcare investor conference in San Francisco in mid-January, a keystone event at which companies often pre-announce guidance and earnings and roll out marquee acquisitions.

From Barron's • Dec. 28, 2025

Last year, some 1,000 U.S. troops withdrew from neighboring Niger, ending a nearly decadelong partnership that was the keystone of U.S. counterterrorism operations in the region.

From The Wall Street Journal • Nov. 2, 2025

The animals are regarded as "keystone" species which can create and maintain habitats for diverse other species.

From BBC • May 12, 2025

Until Columbus, Indians were a keystone species in most of the hemisphere.

From "1491" by Charles C. Mann

Related Words

• base
• cornerstone
• foundation
• fundament