[Erwin]

With databases being often mission critical, the PostgreSQL people decided heroically to support major versions for 5 years -- and as they come out with a new major version every year, minor updates come out for 5 different branches.

Note the recent versioning change: 9.4, 9.5, 9.6 were the previous 3 major versions bases, and the last two are 10 and 11.