Organization of the Registry

The Registry is enormous and complex; a full Registry might easily contain 15,000 keys and 35,000 values. Entire books have been written about it, and I can’t do it justice here. The purpose of this section is to arm you with a basic understanding of how the Registry is organized, not to document individual values in detail or suggest changes you might want to make with the Registry Editor.

The top level of the Registry is organized into five main root branches. By convention, the built-in top-level keys are always shown in all caps, even though the keys in the Registry are not case-sensitive. (For example, HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\Windows is identical to HKEY_CURRENT_USER\Software\Microsoft\Windows.) Their purposes and contents are listed in the following summaries. Note that the root keys are sometimes abbreviated for convenience in documentation (although never in practice); these abbreviations are shown in parentheses. Subsequent sections discuss the contents of the root keys in more detail.


Contains file types, filename extensions, URL protocol prefixes, and registered classes. You can think of the information in this branch as the “glue” that binds Windows with the applications and documents that run on it. It is critical to drag-and-drop operations, context menus, double-clicking, and many other familiar user interface semantics. The actions defined here tell Windows how to react to every file type available ...

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