During the General IRC meeting on 9th March 2016, the developers voted  in a change in the version numbering scheme used in Koha. For almost 3+ years, Koha has been using the version 3.x. Every stable public release had an incrementally updated even number as “x” (i.e. 3.0, 3.2, 3.4… 3.18, 3.20, 3.22 et. al.). The internal development versions (used only by the developers) used the next odd number after the current stable release e.g. 3.21 was used when 3.20.x was the latest stable series.
All that is set to change with the new major version of Koha to be released in just a few days. We are going to align the time-based public releases to yy.mm format. So, the upcoming stable release will be numbered 16.05, and the one scheduled for November 2016 will be 16.11.
References:  http://irc.koha-community.org/koha/2016-03-09#i_1797503