A thumb index, also called a cut-in index, is a round cut-out in the pages of a dictionary

Dictionary

A dictionary is a collection of words in one or more specific languages, often listed alphabetically, with usage information, definitions, etymologies, phonetics, pronunciations, and other information; or a book of words in one language with their equivalents in another, also known as a lexicon...

, encyclopedia

Encyclopedia

An encyclopedia is a type of reference work, a compendium holding a summary of information from either all branches of knowledge or a particular branch of knowledge....

, or other alphabetized reference work, used to locate entries starting at a particular letter

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

. Not all dictionaries and encyclopedias have thumb indexes; their inclusion is considered optional.

Other books which often have thumb indices include Bibles, with the index indicating the books of the Bible, as well as other reference matter.

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