XEmacs and GNU Emacs
Several of XEmacs's principal developers have published accounts of the split between XEmacs and GNU Emacs, for example, Stephen Turnbull's summary of the arguments from both sides. One of the main disagreements involves different views of copyright assignment. The FSF sees copyright assignment to the FSF as necessary to allow it to defend the code against GPL violations, while the XEmacs developers have argued that the lack of copyright assignment has allowed major companies to get involved, as sometimes companies can license their code but due to a cautious attitude concerning fiduciary duties to shareholders, companies may have trouble in getting permission to assign away code completely. The Free Software Foundation holds copyright of much of the XEmacs code because of prior copyright assignment during merge attempts and cross-development. Whether a piece of new XEmacs code enters GNU Emacs often depends on the willingness of that individual contributor to assign the code to the FSF.
New features in either editor usually show up in the other sooner or later. Furthermore, many developers contribute to both projects; in particular, many major Lisp subsystems, such as Gnus and Dired, undergo development to work with both.
Read more about this topic: XEmacs