I learned that in unix, root (e.g. /) is the only directory that is the parent directory of itself.
$ ls -lai / | grep '\./'
2 drwxr-xr-x 20 root wheel 640 Jan 1 2020 ./
2 drwxr-xr-x 20 root wheel 640 Jan 1 2020 ../
In the above example, the files . and .. both have the same i-node: 2
Source: Brian W. Kernighan, & Rob Pike (1984) The UNIX Programming Environment. Prentice-Hall, Inc