GNU EMACS VERSIONING                                   -*- org -*-

Ref: http://lists.gnu.org/archive/html/emacs-devel/2014-09/msg00872.html

Emacs version numbers have the form

  major.minor[.devel].build

"build" increments each time Emacs is built in the same location
(without cleaning) and isn't really part of the version.

bugfix releases increase "minor" by 1.
non-bugfix releases increase "major" by 1, and reset "minor" to 1.
(The division between bugfix and non-bugfix has not always been clear
historically.)

Unreleased (development) versions have an extra "devel" component.
This is a fairly meaningless number that may be unchanged for a long time.
It is normally 50.
When the release process starts, it changes to 90, 91, ...
When the actual release is made, this component is removed.

The development version for a new major release has "minor" = 0.
The development version for a new minor release has "minor" = that of
the previous release.