DIR(5) File Formats Manual DIR(5)
dir - format of directories
A directory behaves exactly like an ordinary file, save that no user
may write into a directory. The fact that a file is a directory is
indicated by a bit in the flag word of its i-node entry see, filsys(5).
The structure of a directory entry as given in the include file is:
By convention, the first two entries in each directory are for `.' and
`..'. The first is an entry for the directory itself. The second is
for the parent directory. The meaning of `..' is modified for the root
directory of the master file system and for the root directories of
removable file systems. In the first case, there is no parent, and in
the second, the system does not permit off-device references. There-
fore in both cases `..' has the same meaning as `.'.