Note: every manifestation of a letter is counted, so the number of documents in a repository may be larger than the number of letters.

Repository Number of documents Further details
Cambridge University Library 244 City: Cambridge
Country: England, United Kingdom
Cardiff Central Library 13 City: Cardiff
Country: Wales, United Kingdom
Central Archives of Historical Records 18 Alternative names for repository: Archiwum Główne Akt Dawnych w Warszawie Archiwum Glowne Akt Dawnych Central Archives of Historical Records, Warsaw
City: Warsaw
Alternative names for city: Warszawa
Country: Poland
Chatsworth House Library 30 Alternative names for repository: Chatsworth House The Devonshire Collection
City: Bakewell, Derbyshire
Country: England, United Kingdom
Cheshire Record Office - Alternative names for repository: Cheshire and Chester Archives and Local Studies Service Cheshire Archives and Local Studies Chester City Record Office
City: Chester
Country: England, United Kingdom
Chetham's Library 67 City: Manchester
Country: England, United Kingdom
Christ Church College Library, University of Oxford 46 City: Oxford
Country: England, United Kingdom
Christ's College Library 9 City: Cambridge
Country: England
Château de Collonges-lès-Bévy 1 City: Collonges-lès-Bévy, Bourgogne
Country: France
Collection d'E. Labrousse 3 Alternative names for repository: Private collection
City: Unknown
College of Arms 2 Alternative names for repository: College of Heralds
City: London
Country: England, United Kingdom
Columbia University, Rare Books and Manuscripts Library 151 City: New York, NY
Country: United States of America
Comenius Museum, Naarden 1 Alternative names for repository: Muzeum Komenského Comenius Museum te Naarden
City: Naarden
Country: Netherlands
Connecticut State Library 7 Alternative names for repository: Connecticut State Archives
City: Hartford, CT
Country: United States of America
Cornwall Record Office - City: Truro
Country: England
Corpus Christi College Library, University of Oxford 12 City: Oxford
Country: England, United Kingdom
Cumbria Archive Centre - City: Carlisle
Country: England