mod.*
Early Usenet's hierarchy for moderated groups, where every post passed through a human before it reached the network.
In the first half of the 1980s nearly everything on Usenet was net.-something and open to all. The handful of groups that had a moderator lived under their own prefix, mod.. Posting to one effectively meant mailing the moderator, who picked, edited and released the articles. The classic case was mod.sources, which distributed source code that somebody had at least looked at first.
The split turned out to be backwards: whether a group was moderated said nothing about what it was about. The Great Renaming of 1986–87 sorted groups by topic instead, and mod.sources became comp.sources.unix. Moderation became a property of the group rather than part of its name.