Special permission bits 

There are three other common permission bits:

         $ ls -la /usr/bin/write
         -rwxr-sr-x  1 root   tty          7540 Jul  4  2002 write

        - --s --s --t 
        |   |   |   |
        |   |   |   \-> sticky bit (t or T)
        |   |   |
        |   |   \-> setgid bit (s or S) - "set group id"
        |   |
        |   \-> setuid bit (s or S) - "set user id"
        |
        \-> Type of file

For example
         -rwxr-sr-x  1 root   tty          7540 Jul  4  2002 write
         -rwxr-sr-x  1 root   utmp       173820 Apr  7  2002 screen
         -rwxr-sr-x  1 root   shadow      17362 Apr  7  2002 xscreensaver
         -rwsr-xr-x  1 root   root        24680 Apr  7  2002 passwd
         -rwsr-xr--  1 root   dip        230604 Dec 10  2001 pppd*