Libraries and librarians have been the gatekeepers to knowledge stores for more than 200 years. As their collections grew, they invented ways to easily find the information and knowledge they stored by creating classification systems and then subject headings to identify the concepts or topics represented in the items being stored. Every major language now has at least one classification system, and most countries have created and adopted classification and subject access systems, such as the Universal Decimal System (UDC) or Lenin’s outline of knowledge for Russia. In the United States, the use of the Dewey Decimal Classification system, Sears Subject Headings, and the Library of Congress Classification system is widespread.