For those whose job or pleasure it is to get English words right, sift their meanings or just generally hit the nail on the head, now is a good time to reflect how much they owe a single book: Roget's Thesaurus.

One hundred and fifty years ago, Peter Mark Roget, a physician and a retired secretary of Britain's Royal Society, published his "Thesaurus of English Words and Phrases Classified and Arranged so as to Facilitate the Expression of Ideas and Assist in Literary Composition." And Feb. 2 this year saw the death at age 81 of Dr. Robert Chapman, the American editor of the fourth and fifth international editions and the man who by general consensus brought "Roget" into the modern world (luckily, earlier editors had already streamlined the title for him). At once solemn and grateful, desolated and beholden, we pause to consider what Roget and his distinguished successor wrought.

Besides boosting forever the cause of those who like their adjectives and adverbs in strings rather than singly or not at all, Roget accomplished several important things with his book. We don't just mean making that quintessential Victorianism, "thesaurus," into a household word. (It's the Latin version of the Greek word for "treasury" or "storehouse," and its plural is "thesauri." But you word lovers already knew all that.) No, Roget's really revolutionary breakthrough was his system of grouping words according to ideas, rather than alphabetically, as in a dictionary. With a dictionary, you know the word and look up its meaning; if the dictionary is a good one, you may also get exact synonyms and examples of usage. Roget, by contrast, came to the aid of all those who had an idea and needed the word for it -- the elusive "word at the tip of one's tongue." With hundreds of synonyms to choose from, covering every possible nuance of a concept, the blocked, the tongue-tied and the absentminded elderly are pretty much assured of help.