While plain file systems directly associate a file name in a directory with the file data, Unix file systems add a layer of indirection between the two, the inodes. The directory entries only store the file name and an inode number; all other data (even the type of the entry) is stored in the inode structure. It is possible to have several directory entries pointing to the same inode. For files, this is called a hard link. For directories, this is used by the file system internally to link back to parent directories. Hard links are somewhat fragile and limited to single file systems. Symbolic links don't link directly to the inode, but store a (usually relative) path, in a separate inode.