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+# How Tor Version Numbers Work
+
+<a id="version-spec.txt-1"></a>
+
+## The Old Way
+
+Before 0.1.0, versions were of the format:
+
+MAJOR.MINOR.MICRO(status(PATCHLEVEL))?(-cvs)?
+
+where MAJOR, MINOR, MICRO, and PATCHLEVEL are numbers, status is one
+of "pre" (for an alpha release), "rc" (for a release candidate), or
+"." for a release.  As a special case, "a.b.c" was equivalent to
+"a.b.c.0".  We compare the elements in order (major, minor, micro,
+status, patchlevel, cvs), with "cvs" preceding non-cvs.
+
+We would start each development branch with a final version in mind:
+say, "0.0.8".  Our first pre-release would be "0.0.8pre1", followed by
+(for example) "0.0.8pre2-cvs", "0.0.8pre2", "0.0.8pre3-cvs",
+"0.0.8rc1", "0.0.8rc2-cvs", and "0.0.8rc2".  Finally, we'd release
+0.0.8.  The stable CVS branch would then be versioned "0.0.8.1-cvs",
+and any eventual bugfix release would be "0.0.8.1".
+
+<a id="version-spec.txt-2"></a>
+
+## The New Way
+
+Starting at 0.1.0.1-rc, versions are of the format:
+
+`MAJOR.MINOR.MICRO[.PATCHLEVEL][-STATUS_TAG][ (EXTRA_INFO)]*`
+
+The stuff in parentheses is optional.  As before, MAJOR, MINOR, MICRO,
+and PATCHLEVEL are numbers, with an absent number equivalent to 0.
+All versions should be distinguishable purely by those four
+numbers.
+
+The STATUS_TAG is purely informational, and lets you know how
+stable we think the release is: "alpha" is pretty unstable; "rc" is a
+release candidate; and no tag at all means that we have a final
+release. If the tag ends with "-cvs" or "-dev", you're looking at a
+development snapshot that came after a given release.  If we *do*
+encounter two versions that differ only by status tag, we compare them
+lexically. The STATUS_TAG can't contain whitespace.
+
+The EXTRA_INFO is also purely informational, often containing information
+about the SCM commit this version came from. It is surrounded by parentheses
+and can't contain whitespace. Unlike the STATUS_TAG this never impacts the way
+that versions should be compared. EXTRA_INFO may appear any number of
+times. Tools should generally not parse EXTRA_INFO entries.
+
+Now, we start each development branch with (say) 0.1.1.1-alpha.  The
+patchlevel increments consistently as the status tag changes, for
+example, as in: 0.1.1.2-alpha, 0.1.1.3-alpha, 0.1.1.4-rc, 0.1.1.5-rc.
+Eventually, we release 0.1.1.6.  The next patch release is 0.1.1.7.
+
+Between these releases, CVS is versioned with a -cvs tag: after
+0.1.1.1-alpha comes 0.1.1.1-alpha-cvs, and so on. But starting with
+0.1.2.1-alpha-dev, we switched to SVN and started using the "-dev"
+suffix instead of the "-cvs" suffix.
+
+<a id="version-spec.txt-3"></a>
+
+## Version status
+
+```text
+  Sometimes we need to determine whether a Tor version is obsolete,
+  experimental, or neither, based on a list of recommended versions.  The
+  logic is as follows:
+
+   * If a version is listed on the recommended list, then it is
+     "recommended".
+
+   * If a version is newer than every recommended version, that version
+     is "experimental" or "new".
+
+   * If a version is older than every recommended version, it is
+     "obsolete" or "old".
+
+   * The first three components (major,minor,micro) of a version number
+     are its "release series".  If a version has other recommended
+     versions with the same release series, and the version is newer
+     than all such recommended versions, but it is not newer than
+     _every_ recommended version, then the version is "new in series".
+
+   * Finally, if none of the above conditions hold, then the version is
+     "un-recommended."
+```