DocBook originally started out as an SGML application, but an equivalent XML application was developed and has now replaced the SGML one for most uses. (Starting with version 4 of the SGML DTD, the XML DTD continued with this version numbering scheme.) Initially, a key group of software companies used DocBook since their representatives were involved in its initial design. Eventually, however, DocBook was adopted by the open source community where it has become a standard for creating documentation for many projects, including FreeBSD, KDE, GNOME desktop documentation, the GTK+ API references, the Linux kernel documentation, and the work of the Linux Documentation Project.