Other forms: keystones
The keystone is the wedge-shaped stone at the very top of an arch, the last one dropped in and the one that locks all the others in place. Look up at any old stone bridge and it is the block in the middle, the one carved with a date.
The name is literal: key in its old sense of the thing that fastens, plus stone. The figurative use is the common one now, as in the keystone of the policy or the keystone of the whole argument, meaning the piece everything else rests on. Ecologists borrowed it for keystone species, an animal or plant whose loss brings a whole habitat down, such as the sea otter or the beaver. Keep it apart from cornerstone, which is the first stone laid and the foundation of a thing, while the keystone goes in last and holds the rest together.
1nounkeystoneThe wedge-shaped stone at the top of an arch, which locks the other stones in place.~36%
“The keystone of the bridge carries the date 1743.”
2nounlinchpinThe part of a plan, argument or system that everything else depends on.~64%
“Free school meals were the keystone of the whole programme.”
synonyms: linchpin, cornerstone
Shares are an estimate: a language model read 11 sentences found in the wild and placed 11 of them on a sense — a count of our sample, not a measurement of English.
keystone is a building stone.
Keystone is key plus stone, and the key here is the older meaning of the word: the thing that fastens or locks. The stone at the crown of an arch does exactly that, being the last piece in and the one that stops the others sliding. The metal key in your pocket is the same idea applied to a door.
From key + stone. Definition 4 (retail) possibly originated in the jewelry industry in the magazine called "Jewelers' Circular-Keystone".
Etymology from Wiktionary, CC BY-SA