The repository format is compatible going back to
cvs 1.3. But see Using watches with old versions of CVS, if
you have copies of CVS 1.6 or older and you want
to use the optional developer communication features.
The working directory format is compatible going back
to CVS 1.5. It did change between CVS 1.3
and CVS 1.5. If you run CVS 1.5 or newer on
a working directory checked out with CVS 1.3,
CVS will convert it, but to go back to CVS
1.3 you need to check out a new working directory with
The remote protocol is interoperable going back to CVS 1.5, but no
further (1.5 was the first official release with the remote protocol,
but some older versions might still be floating around). In many
cases you need to upgrade both the client and the server to take
advantage of new features and bug fixes, however.