At a glance, it’s obvious that the Historical Thesaurus of the Oxford English Dictionary, published in the fall to worldwide lexicographic acclaim, took a lot of work. The two thick blue volumes sort roughly 920,000 words into 797,120 meanings and organize those meanings into 236,400 numbered categories and subcategories. Like Peter Mark Roget’s famous thesaurus, first issued in 1852, the H.T.O.E.D. not only lists synonyms but also provides what its editors call a “modified folk taxonomy” of English’s meanings — carefully placing deity, for example, among the supernatural elements of the external world, while slotting worship among the manifestations of faith in the social world. It draws on the second edition of the Oxford English Dictionary, as well as a few Old English sources. The task of dicing up the O.E.D. and reassembling it into the H.T.O.E.D. took 44 years, and most of the intellectual labor was performed by humans, working with pencils and slips of paper. A 1978 fire would have proved disastrous if the slips had not been stored in metal drawers in metal filing cabinets.

It’s not immediately apparent, however, what the H.T.O.E.D. is for. Like Roget’s, it could be used by a hack to tart up his prose. Indeed, consulting the two volumes, an inferior writer may come to know himself by the names paper-blurrer, feather-driver, blotter, squitter-pulp, spoil-paper and penster. Unfortunately, no one uses these names any more, and rare is the editor indulgent enough to permit an ink-dabbler to be indifferent to currency.

But archaism, it turns out, is the point of the H.T.O.E.D., which includes outmoded words as well as contemporary ones and indicates when words came into and fell out of use. The equivalents for “inferior writer” mentioned above, for instance, all flourished in Shakespeare’s day. Did the golden age of English poetry somehow call forth a treasury of antiwriter invective? The H.T.O.E.D. is designed to occasion such speculations. What does it mean that in Old English there were 40 words for the Christian God in his capacity as ruler (e.g., wundorcyning), whereas today we have just six (e.g., the Almighty)? Why were there 44 ways to call someone wise in Old English, but we can add only that he is sage or judicious?

Whereas a dictionary makes it possible to follow the history of a word, a historical thesaurus — the H.T.O.E.D. claims to be the world’s first, in any language — makes it possible to follow the history of a meaning. It’s like watching an actor try on new costumes and shed old ones, or like cruising down a river that in one stretch narrows to a rapids and at another broadens to a marsh. With a little effort, a historical thesaurus can even serve as a vehicle for a kind of linguistic time travel. “For any given period in the past,” the editors write, “the user should be able to ascertain the exact state of the vocabulary (i.e., the ‘lexical system’) which existed at that time.”