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The Google Ngram Viewer is a phrase-usage graphing tool which charts the yearly count of selected n-grams (letter combinations),[n] words, or phrases, as found in over 5.2 million books digitized by Google Inc (up to 2008). The words or phrases (or ngrams) are matched by case-sensitive spelling, comparing exact uppercase letters, and plotted on the graph if found in 40 or more books. The Ngram tool was released in mid-December 2010.
The word-search database was created by Google Labs, based originally on 5.2 million books, published between 1500 and 2008, containing 500 billion words in American English, British English, French, German, Spanish, Russian, or Chinese. Italian words are counted by their use in other languages. A user of the Ngram tool has the option to select among the source languages for the word-search operations.
The Ngram Viewer returns a plotted line chart within seconds of listing entries and pressing the Enter key or the "Search" button on the screen. The entries are separated by commas, to indicate each separate word or phrase to find. As an adjustment for more books having been published during some years, the data is normalized, as a relative level, by the number of books published in each year.
The database was created from over 5 million books published up to 2008, and so, as of May 2012, no data will match beyond the year 2008. Due to limitations on the size of the Ngram database, only matches found in over 40 books are indexed in the database; otherwise, not all known combinations could have been stored.
Typically, entries cannot end with punctuation, although a separate full stop, or period, can be searched. Also, an ending question mark (as in "Why?") will cause a 2nd search for the question mark separately.
Omitting the periods in abbreviations will allow a form of matching, such as using "R M S" to search for "R.M.S." versus "RMS".