(Excerpt from docbook-5.0/rng/docbook.rng)

Copyright 1992-2008 HaL Computer Systems, Inc.,
O'Reilly & Associates, Inc., ArborText, Inc., Fujitsu Software
Corporation, Norman Walsh, Sun Microsystems, Inc., and the
Organization for the Advancement of Structured Information
Standards (OASIS).

Permission to use, copy, modify and distribute the DocBook schema
and its accompanying documentation for any purpose and without fee
is hereby granted in perpetuity, provided that the above copyright
notice and this paragraph appear in all copies. The copyright
holders make no representation about the suitability of the schema
for any purpose. It is provided "as is" without expressed or implied
warranty.

If you modify the DocBook schema in any way, label your schema as a
variant of DocBook. See the reference documentation
(http://docbook.org/tdg5/en/html/ch05.html#s-notdocbook)
for more information.

Please direct all questions, bug reports, or suggestions for changes
to the docbook@lists.oasis-open.org mailing list. For more
information, see http://www.oasis-open.org/docbook/.