A page often has several authors. We expect each author's edits to be an improvement.
But, just to be safe, we save a copy of the previous author's text.
The EditText page knows when a backup copy is available and
will offer the EditCopy hyperlink to access it.
From this page you can...
Save the copy as the desired version
Edit the copy and save that
Cut text from the copy to use elsewhere
See a diff of the two versions
A new backup copy is made whenever a page is saved by other than the most recent author.
The current implementation uses the host machine name to detect change of authorship.
Two authors on the same machine (or behind a firewall) appear as one; one author using different hosts or dynamic IP addresses appears as two.
Older versions are kept for a while in http:/wiki/history. See HistoryPages for an explanation, and links to deeper archives.
See also HistoryPages, RevertPage, VersionHistory, EditCopy and PerpetualNow.