[begin work on 0.4 branch]
       |
       |
       v
(*)---(*)-------(a)--(b)---(c)-- <-- Trunk is "unstable".
                |          |        Contains all commits.
    ver      [merge from trunk]     Developers commit to trunk.
<-- 0.3         v          v
            +---(a)--------(c)-- <-- Branch is "stable".
                                     Contains selected commits from trunk.
                                     Know beforehand what's going onto branch.

[trunk]
(*)---(*)---(*)----------------------------[development continues]--->


[0.4 branch]                        No further development on branch unless
(*)---(*)---(*)---[0.4-release]     spot fixes are needed. Then re-tag (0.4.1)
                          ^         and re-release.
                          |
                          |
                       [make tag on branch; release from stable branch,
                        not unstable trunk]

- Project
    |-  development mainline
    |-  TAG: version-1.0
         |- version-1.0-bugfix#123
         |- version-1.0-bugfixes
    |-  TAG: version-1.0-sp1
         |- version-1.0-sp1-bugfix#234
         |- version-1.0.sp1-bugfixes
    |-  TAG: version-1.0-sp2