- Two directory entries that point to the same file (inode)
- to delete the file, must delete both hard links
- must exist on a single low-level file system
Ordinarily, you can’t create hard links to directories. The root user can attempt to do so, though, by passing the -d, -F, or –directory option to ln.
(Symbolic links to directories aren’t a problem.)
In practice, this feature is unlikely to work because most filesystems don’t support it.