Presidential libraries aren't libraries in the traditional sense. They're much more: Repositories for preserving the papers, records, collections and other historical materials of every U.S. president since Calvin Coolidge. They're also museums that include permanent and temporary exhibits, seminars and seminars and gift shops.

The presidential library system started in 1939 when Franklin D. Roosevelt donated his personal and professional papers to the federal government, as well as offered his Hyde Park estate as a place to preserve them. Roosevelt's friends formed a non-profit corporation to raise funds for the construction of both the library and museum building.

His decision to make the donation stemmed from his belief that presidential papers were an important part of the national heritage and should be accessible to the public.

Before Roosevelt, there had been no formal method for keeping presidential papers. In 1950, Harry S. Truman announced he would also build a library; five years later Congress passed the Presidential Libraries Act, establishing a system of privately built, but federally maintained, libraries. Other presidents were encouraged to donate their historical materials, ensuring that presidential papers would be available to the American people.

In addition to Roosevelt's and Truman's, the other official libraries are for George W. Bush (construction is planned to begin in 2009), Bill Clinton, George H. W. Bush, Ronald Reagan, Jimmy Carter, Gerald Ford, Richard Nixon, Lyndon Johnson, John F. Kennedy and Dwight Eisenhower. The Libraries Act grandfathered the Herbert Hoover library into the system.

Johnson's library has the most visitors per year, at about 200,000; Jimmy Carter's has the second-highest number of visitors.

The presidential libraries maintain more than 400 million pages of textual materials; almost 10 million photographs; more than 15 million feet of motion picture film; and nearly 100,000 hours of disc, audiotape and videotape recordings.

The libraries house objects such as campaign memorabilia, awards, family heirlooms, items collected by the president and his family, in addition to the many items given to the president by American citizens and foreign dignitaries.

The most important part within each library is the textual materials created by the president and his staff while performing official duties.

Other significant holdings include the personal papers and materials donated by individuals who were associated with the president. Those individuals included cabinet officials, political party associates, envoys to foreign governments and the president's family and close friends.

Many pre-Hoover collections can be found in the Library of Congress. Before the presidential library system was created, presidents and/or their heirs often spread out their presidential papers at the end of an administration, split among other libraries, historical societies and private collections. Many of the materials have been lost or deliberately destroyed.

The presidential library system is a nationwide network of libraries administered by the Office of Presidential Libraries, which is part of the National Archives and Records Administration.

With the exception of Kennedy, Johnson and Carter, every American president since Hoover is or has chosen to be buried at their presidential library.