Concurrent Versions System
Concurrent Versions System (CVS) is an open-source revision control system for software development, dating back to the 1980s (with the current CVS descending from a 1990 release of "version 1.0", though there were earlier versions with a different code base). It used to be very popular for open-source projects, though in more recent times Git is more popular. It is not to be confused with the CVS drug store chain, or with CSV files.
CVS uses files with names ending in
,v, which is not a normal file extension since it has a comma instead of a period in it.