A dictionary is a collection of words and their definitions, often organized alphabetically. It can also refer to a book that contains such a collection. The first known dictionaries date back to Sumerian times in the 3rd millennium BC, but the modern dictionary was not developed until the 16th century. It is an essential reference tool for language learners of any age, as it provides comprehensive information on words and their meanings. A dictionary typically includes the following: pronunciation, part of speech, definition, etymology (origin), synonyms (words with similar meanings), antonyms (words with opposite meanings), usage notes (such as colloquialisms or regionalisms), example sentences, and related words. Depending on its purpose and audience, a dictionary may also include other features such as illustrations, pronunciation guides, verb conjugations, word origins and histories, usage statistics and quotations from literature. Dictionaries are invaluable resources for anyone who needs to find out more about language: writers and editors use them to check spelling; teachers use them to teach vocabulary; lawyers use them to understand legal terms; students use them for researching assignments; linguists use them for language study; translators use them for translation work; and readers in general use them to look up unfamiliar words or phrases.