Archival materials are also known as primary sources. The records held by archives are unique and irreplaceable. By their very nature archival materials are fragile and vulnerable to improper handling. If an archival document is lost, stolen, or irreparably damaged, the information it contains is lost forever. Published material is also known as secondary sources. The holdings of one library may be duplicated in the holdings of another. If a book is lost or stolen it probably can be replaced.