http://kor.i2p/books/The%20C%20Programming%20Language%20by%20K&R/chapter8.html
In the UNIX file system,
there are nine bits of permission information associated with a file that
control read, write and execute access for the owner of the file, for the
owner's group, and for all others. Thus a three-digit octal number is
convenient for specifying the permissions. For example, 0775 specifies
read, write and execute permission for the owner, and read and execute
permission for the group and everyone else.