Writers often turn to a thesaurus to diversify their vocabulary and add nuance to their prose. But looking up synonyms and antonyms in a thesaurus can help anyone—writer or not—find the most vivid, incisive words to communicate thoughts and ideas. Since January 18 is Thesaurus Day, we’re celebrating with these 10 fascinating facts about your thesaurus.

1. Thesaurus comes from the Greek word for treasure.

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Most logophiles consider the thesaurus to be a treasure trove of diction, but the word thesaurus really does mean "treasure." It derives from the Greek word thésauros, which means a storehouse of precious items, or a treasure.

2. The plural of thesaurus is thesauruses or thesauri.

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How do you refer to more than one octopus? People say everything from octopuses to octopi to octopodes. Similarly, many people have trouble figuring out the correct plural form of the word thesaurus. Though thesauri is technically correct—it attaches a Latin suffix to the Latin word thēsaurus—both thesauri and thesauruses are commonly used and accepted today.

3. Early thesauruses were really dictionaries.

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Ask a French scholar in the 16th century to see his thesaurus, and he'd gladly give you a copy of his dictionary. In the early 1530s, a French printer named Robert Estienne published Thesaurus Linguae Latinae, a comprehensive Latin dictionary listing words that appeared in Latin texts throughout an enormous span of history. And in 1572, Estienne's son Henri published Thesaurus Linguae Graecae, a dictionary of Greek words. Although the Estiennes's books were called thesauruses, they were really dictionaries comprised of alphabetical listings of words with their definitions.

4. A Greek historian wrote the first book of synonyms.

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Philo of Byblos, a Greek historian and grammarian, wrote On Synonyms, a dictionary of synonyms that scholars consider to be the first ancient thesaurus. Dating to the late 1st century or early 2nd century CE, the book lists Greek words that are similar in meaning to each another. Sadly, we don’t know much more about On Synonyms because copies of the work haven’t survived over the centuries.

5. An early Sanskrit thesaurus was written in the form of a poem.

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In the 4th century CE, an Indian poet and grammarian named Amara Sinha wrote The Amarakosha, a thesaurus of Sanskrit words. Rather than compile a boring list of similar words, Amara Sinha turned his thesaurus into a long poem. Divided into three sections—words relating to the divine, the earth, and everyday life—The Amarakosha contains verses so readers could memorize words easily. This thesaurus is the oldest book of its kind that still exists.

6. A British doctor wrote the first modern thesaurus.

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Peter Mark Roget is the British doctor credited with authoring the first modern thesaurus. In 1805, he began compiling a list of words, arranged by their meaning and grouped according to theme. After retiring from his work as a physician in 1852, Roget published his Thesaurus of English words and phrases; so classified and arranged as to facilitate the expression of ideas and assist in literary composition. Today, Roget’s Thesaurus is still commercially successful and widely used. In fact, we celebrate Thesaurus Day on January 18 because Roget was born on this day in 1779.

7. The thesaurus has a surprising link to a mathematical tool.

The division between "words people" and "numbers people" is deep-seated. Many mathematicians may try to steer clear of thesauruses, and bibliophiles may avoid calculators, but the thesaurus is actually linked to a mathematical tool. Around 1815, Roget invented the log-log slide rule, a ruler-like device that allows users to easily calculate the roots and exponents of numbers. So while the inventor of the thesaurus was compiling words for his tome, he was also hard at work on the log-log slide rule. A true jack-of-all-trades.

8. The Oxford English Dictionary has its own historical thesaurus.

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In 1965, a professor of English Language at Glasgow University suggested that scholars should create a historical thesaurus based on entries in the Oxford English Dictionary. The project was a massive undertaking, as people from multiple countries worked for 44 years to compile and classify words. Published in 2009, the Historical Thesaurus to the Oxford English Dictionary contains 800,000 words organized by theme and date. The thesaurus covers words and synonyms from Old English to the present day and lets readers discover when certain words were coined and how long they were commonly used.

9. One artist turned his love of words into a series of thesaurus paintings.

In 2014, the Jewish Museum in New York showed a survey of conceptual artist Mel Bochner’s art. Bochner had incorporated words and synonyms in his paintings for years—which were collectively referred to as the thesaurus paintings—featuring word paintings and lists of synonyms on canvas. The brightly colored paintings feature different groups of English and Yiddish synonyms. According to Bochner, Vietnam and Iraq war veterans cried after seeing his thesaurus painting Die, which features words and phrases such as expire, perish, succumb, drop dead, croak, go belly up, pull the plug, and kick the bucket.

10. There's an urban thesaurus for all your slang synonym needs.

Urban Dictionary helps people decipher the latest slang terms, but where should you go when you need a thesaurus of slang? Urban Thesaurus, of course. The site, which is not affiliated with Urban Dictionary, indexes millions of slang terms culled from slang dictionaries, then calculates usage correlations between the terms. Typing in the word money, for example, gives you an eclectic list of synonyms including scrilla, cheddar, mulah, coin, and bling.