The Google Ngram Viewer is an online phrase-usage graphing tool originally developed by Google, inspired by a prototype (called "Bookworm") created by Jean-Baptiste Michel and Erez Aiden from Harvard and Yuan Shen from MIT. It charts the yearly count of selected n-grams (letter combinations)[n] or words and phrases, as found in over 5.2 million books digitized by Google Inc (up to 2012). 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 during each year (of the requested year-range). The Ngram tool was released in mid-December 2010 and now supports searches for parts of speech and wildcards.
The word-search database was created by Google Books and was based originally on 5.2 million books published between 1500 and 2008. Collectively, the corpus contained over 500 billion words in American English, British English, French, German, Spanish, Russian, Hebrew, and 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.
Researchers have analyzed the Google Ngram database of books written in American or British English discovering interesting results. Amongst them, they found correlations between the emotional output and significant events in the 20th century such as the World War II.