parent directories of directories

[Jonas Bernoulli]

How does one get the parent directory of a directory in a way that
works on all supported platforms?

For regular files `file-name-directory` can be used but for
directories this just returns the given argument.

One could use `(file-name-directory (substring 0 -1 directory))` but I
have no idea wether this works on all platforms.

Thanks

Jonas