Link: The Single Unix Specification et al
The Open Group Base Specification Issue 6 is, well, to quote it:
This standard is the single common revision to IEEE Std 1003.1-1996, IEEE Std 1003.2-1992, and the Base Specifications of The Open Group Single UNIX Specification, Version 2.
(Those IEEE standards are better known as 'POSIX'.)
The Single Unix Specification (SUS) is a very useful authoritative reference for how various things should behave in theory. (How they behave in fact is a different issue; not everything is correctly implemented, and not everything is SUS/POSIX compliant to start with.)
You'd think that the Open Group would make this stuff openly available, but instead they want you to register and provide them with various personal details. As we can see here, that is not strictly speaking necessary; you just need the right magic URL.
(From Andree Leidenfrost via Debian Planet.)