CVS (Concurrent Versioning System) is the standard tool for allowing several developers or authors to work together on a shared set of documents. CVS maintains a central repository of all files associated with a project, including a history of all changes made to each file. Users checkout copies of the central repository (either on to a local machine or else any machine on the network), edit documents locally, then commit changes back to the repository. Multiple users can be working on the same project concurrently. All older versions of the documents can be retrieved, if required.