This is the not-entirely-unique practice of using odd minor numbers for beta releases and even minor numbers for stable releases. Gnome does this as well - 3.1

This is the not-entirely-unique practice of using odd minor numbers for beta releases and even minor numbers for stable releases. Gnome does this as well - 3.17.x was beta, 3.18.x stable. Linux was one of the first projects to use this pattern, but hasn't since early 2.6.x.