The Royal Commission on Historical Manuscripts was created in 1869 with the purpose of locating and providing accessibility to researchers records held in private collections, such as by individuals, families, or institutions. The Commission does not contain any of these records, but publishes reports on them, providing details such as what type of information is contained in the manuscripts and where the manuscripts are located. This database contains the Fifth Report published by the Commission. A variety of record types are described in this work. A contents page located at the beginning of the publication may help you determine if this is a work you might be interested in. In April 2003 the Historical Manuscripts Commission and the Public Records Office were brought together to form the National Archives of the United Kingdom.
Some of the above information was taken from The Dictionary of Genealogy by Terrick V. H. Fitzhugh; (London: A and C Black, 1994).