CVS was not of the exclusive-check-out/check-in kind, it would merge by default on "cvs update", this was one of the main differences between it and RCS or sccs. The other main difference was it had a network protocol and didn't rely on the filesystem for sharing (e.g. NFS and the like). It was probably this 2nd difference that enabled it, (and subversion), to overtake RCS and sccs in the 90s.