LinuxDoc is an SGML DTD which is similar to DocBook.
Matt Welsh created it, and version 1.1 was announced [1] in 1994.
[2] LinuxDoc has a more succinct DTD than DocBook.
Users of small to medium-size projects have found that it suits their purposes better than DocBook.
[3] The Debian distribution of Linux has a Linuxdoc-tools package.