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(emacs)VC Concepts

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Concepts of Version Control
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   When a file is under version control, we also say that it is
"registered" in the version control system.  Each registered file has a
corresponding "master file" which represents the file's present state
plus its change history--enough to reconstruct the current version or
any earlier version.  Usually the master file also records a "log
entry" for each version, describing in words what was changed in that
version.

   The file that is maintained under version control is sometimes called
the "work file" corresponding to its master file.  You edit the work
file and make changes in it, as you would with an ordinary file.  (With
SCCS and RCS, you must "lock" the file before you start to edit it.)
After you are done with a set of changes, you "check the file in",
which records the changes in the master file, along with a log entry for
them.

   With CVS, there are usually multiple work files corresponding to a
single master file--often each user has his own copy.  It is also
possible to use RCS in this way, but this is not the usual way to use
RCS.

   A version control system typically has some mechanism to coordinate
between users who want to change the same file.  One method is
"locking" (analogous to the locking that Emacs uses to detect
simultaneous editing of a file, but distinct from it).  The other method
is to merge your changes with other people's changes when you check them
in.

   With version control locking, work files are normally read-only so
that you cannot change them.  You ask the version control system to make
a work file writable for you by locking it; only one user can do this
at any given time.  When you check in your changes, that unlocks the
file, making the work file read-only again.  This allows other users to
lock the file to make further changes.  SCCS always uses locking, and
RCS normally does.

   The other alternative for RCS is to let each user modify the work
file at any time.  In this mode, locking is not required, but it is
permitted; check-in is still the way to record a new version.

   CVS normally allows each user to modify his own copy of the work file
at any time, but requires merging with changes from other users at
check-in time.  However, CVS can also be set up to require locking.
(Note: CVS Options).