Usage History of 'keystone'

The following chart shows how frequently 'keystone' appeared in printed materials over time. For example, the word 'keystone' occurred 4 times in books in 1590 and 40,720 times in 2008. From 1590 to 2008, the word 'keystone' was used a total of 1,251,423 times in books scanned by Google. See Reference: Google Ngram project.

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