In short, RCS maintains a clean copy of the head revision, and a set of reverse patches to be applied to recreate older revisions.  SCCS maintains a sequence of blocks of lines that were added or deleted at the same time, and any revision can be extracted in the same amount of time by scanning the blocks and retaining those that are pertinent.<p>Really old school revision control systems, like CDC&#x27;s MODIFY and Cray&#x27;s clone UPDATE, were kind of like SCCS.  Each line (actually card image!) was tagged with the ids of the mods that created and (if no longer active) deleted it.