Windows drive letters are not limited to A-Z

Windows drive letters are not limited to A-Z

2025-11-30

On its own, the title of this post is just a true piece of trivia, verifiable with the built-in subst tool (among other methods).

Here's an example creating the drive +:\ as an alias for a directory at C:\foo :

subst +: C:\foo

The +:\ drive then works as normal (at least in cmd.exe, this will be discussed more later):

> cd /D +:\ +:\> tree . Folder PATH listing Volume serial number is 00000001 12AB:23BC +:\ └───bar

However, understanding why it's true elucidates a lot about how Windows works under the hood, and turns up a few curious behaviors.

What is a drive letter, anyway?🔗

The paths that most people are familiar with are Win32 namespace paths, e.g. something like C:\foo which is a drive-absolute Win32 path. However, the high-level APIs that take Win32 paths like CreateFileW ultimately will convert a path like C:\foo into a NT namespace path before calling into a lower level API within ntdll.dll like NtCreateFile .

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