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Based on the Oxford English Dictionary, the Historical Thesaurus is the largest thesaurus in the world. Over forty years in the making, it contains almost every word in the English language from Old English to the present, allowing readers to explore words connected in meaning throughout the history of the language.

The Historical Thesaurus is divided into two volumes: a browsable main text thesaurus, and an index to find words that through the ages have been similar to it. It contains over 800,000 meanings organised into more than 236,000 semantic categories and subcategories in a descending hierarchy indicated by a numeric classification system.