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CliffsNotes (formerly Cliffs Notes, originally Cliff's Notes and often, erroneously, CliffNotes) are a series of student study guides available primarily in the United States. The guides present and explain literary and other works in pamphlet form or online. Detractors of the study guides claim they let students bypass reading the assigned literature. The company claims to promote the reading of the original work, and does not view the study guides as a substitute for that reading.
CliffsNotes was started by a Nebraska native named Cliff Hillegass in 1958. He was working at Nebraska Book Co. of Lincoln, Nebraska, when he met Jack Cole, the co-owner of Coles, a Toronto book business. Coles was also the publisher of a series of Canadian study guides called Coles Notes. Jack Cole offered the American rights to Hillegass.
Hillegass and his wife, Catherine, started the business in their basement at 511 Eastridge Drive in Lincoln, with sixteen William Shakespeare titles. CliffsNotes now exist on hundreds of works. The term "Cliff's Notes" has now come into modern usage as a generic noun for similar products.
IDG Books purchased CliffsNotes in 1998. John Wiley & Sons acquired IDG Books (renamed Hungry Minds) in 2001. In 2011, CliffsNotes announced a joint venture with AOL and reality TV show producer Mark Burnett to introduce a series of 60-second video study guide surveys of literary works.  In 2012, Houghton Mifflin Harcourt (HMH) acquired CliffsNotes, Inc., and the brand lives on today as part of the global learning company, and its mission of changing lives by fostering passionate, curious learners.
In addition to guides for literature, the company produces several other series of guides, including:
- CliffsQuickReview: A quick reference for information on scholastic subjects, such as sciences, mathematics, and history.
- CliffsComplete: Similar to regular CliffsNotes, but also includes notes and commentary.
- CliffsAP: Advanced placement study guides, including action plans, test-taking strategies, reviews, and exercises.
- CliffsStudySolver: A learn-by-mastering approach to algebra, Spanish, grammar, and other subjects.
- CliffsTestPrep: Test preparation guides for the LSAT, SATs, GED, and other tests.
- Shakespeare on the Double!: The original Shakespeare text side-by-side with a contemporary English version.
- The Manga Editions: Based on the traditional manga format.