Just after I was done writing Managing PATH and MANPATH, I stumbled on “man man” and put to rest the mysteries of MANPATH.
How it works
If MANPATH is defined, it will be used to lookup man pages.
If MANPATH is NOT defined, the manpath config file is going to be used. Depending on the OS your are using, it might be something like /etc/man.conf (Mac OS X) or /etc/manpath.config (Ubuntu).
Here’s an excerpt for “man man” (Mac OS X):
It says there’s a command line flag to override MANPATH (-M), but I don’t think that’s excessively useful.
Here’s something useful: when you aren’t sure what man page you’re going to get, try the -w flag.