Pillow follows Semantic Versioning:
Given a version number MAJOR.MINOR.PATCH, increment the:
MAJOR version when you make incompatible API changes,
MINOR version when you add functionality in a backwards compatible manner, and
PATCH version when you make backwards compatible bug fixes.
Quarterly releases (“Main Release”) bump at least the MINOR version, as new functionality has likely been added in the prior three months.
A quarterly release bumps the MAJOR version when incompatible API changes are made, such as removing deprecated APIs or dropping an EOL Python version. In practice, these occur every 12-18 months, guided by Python’s EOL schedule, and any APIs that have been deprecated for at least a year are removed at the same time.
Between quarterly releases, “.dev0” is appended to the “master” branch, indicating that this is not a formally released copy.