File owner must belong to file group?

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http://unix.stackexchange.com – I have a rather simplistic understanding of file permissions on *nix systems. I understand that there is a file owner and file group, but is there a hard and fast rule on whether the said file owner must belong to the file group also? Or put another way, can a file belong to a group that the owner is not a part of? If so (or if not), why? I'd like to increase my understanding... I can't seem to find anything that specifically talks about this out on the interwebs... I'm also open to some good reading material on the subject. Thanks. (HowTos)