According to the POSIX specification, paths that begin with *exactly two slashes* have implementation-defined semantics.

Multiple slashes in a path are collapsed to one slash, *except* for the case where there is *exactly two* slashes at *exactly the beginning* of the path.

So, `/foo`, `///foo`, `////foo`, and `///////////////////foo` are guaranteed to be the same path, `/foo/bar`, `/foo//bar`, `/foo///bar`, and so on are guaranteed to be the same path, but `/foo` and `//foo` are *not* guaranteed to be the same path, and neither are `//foo` and `///foo` – any implementation can define the semantics as they wish. They *could* choose to define `//foo` to mean the same thing as `/foo` and `///foo`, but they don't *have to*.

The intention is that operating systems can use paths beginning with `//` to implement semantics that are different from POSIX file system semantics.

For example, a hypothetical hybrid of Windows and Unix could use `//` for Windows filesystem semantics. Cygwin actually uses `//` for network paths, similar to how Windows uses `\\`.

In Cygwin, for example, `cd //; ls` would list all SMB fileserver on the local network, not the root directory!