In this key, the Registry entries define types (or classes) of documents and the properties associated with those types. Both conventional applications and OLE applications use data that is stored under this key. This key also provides backward compatibility with the Windows 3.1 registration database by storing information for DDE and OLE support. File viewers and user interface extensions store their OLE class identifiers in HKEY_CLASSES_ROOT, and in-process servers are registered in this key. (You'll find more information about in-process servers and how they work with file viewers and user interface extensions in Chapters 12 and 13.)