as it is represented by a numeral: The read bit adds 4 to its total (in binary 100), The write bit adds 2 to its total (in binary 010), and The execute bit adds 1 to its total (in binary 001). These values never produce ambiguous combinations; each sum represents a specific set of permissions. More technically, this is an octal representation of a bit field – each bit references a separate permission, and grouping 3 bits at a time in octal corresponds to grouping these permissions by user, group, and others. These are the examples from the symbolic notation section given in octal notation: User private group Some systems diverge from the traditional POSIX model of users and groups by creating a new group – a "user private group" – for each user. Assuming that each user is the only member of its user private group, this scheme allows an umask of 002 to be used without allowing other users to write to newly created files in normal directories because such files are assigned to the creating user's private group. However, when sharing files is desirable, the administrator can create a group containing the desired users, create a group-writable directory assigned to the new group, and, most importantly, make the directory setgid. Making it setgid will cause files created in it to be assigned to the same group as the directory and the 002 umask (enabled by using user private groups) will ensure that other members of the group will be able to write to those files. See also Comparison_of_file_systems#Metadata chmod: change mode (permissions) on Unix-like file systems chattr or chflags: change attributes or flags including those which restrict access. lsattr list attributes POSIX umask User identifier (Unix) Group identifier (Unix) References External links The Linux Cookbook: Groups and How to Work in Them by Michael Stutz 2004