--- a/devel-versioning.rst	Sun Nov 27 23:45:17 2011 +0200
+++ b/devel-versioning.rst	Sun Nov 27 23:50:15 2011 +0200
@@ -208,7 +208,100 @@
 Debian version ordering formula.
 --------------------------------
 
-TODO
+The version number of a package.  The format is::
+
+  [<epoch>`:']<upstream_version>[`-'<debian_revision>]
+
+The three components here are:
+
+``epoch``
+     This is a single (generally small) unsigned integer.  It may be
+     omitted, in which case zero is assumed.  If it is omitted then
+     the <upstream_version> may not contain any colons.
+
+     It is provided to allow mistakes in the version numbers of older
+     versions of a package, and also a package's previous version
+     numbering schemes, to be left behind.
+
+``upstream_version``
+     This is the main part of the version number.  It is usually the
+     version number of the original ("upstream") package from which
+     the ``.deb`` file has been made, if this is applicable.  Usually
+     this will be in the same format as that specified by the upstream
+     author(s); however, it may need to be reformatted to fit into the
+     package management system's format and comparison scheme.
+
+     The comparison behavior of the package management system with
+     respect to the <upstream_version> is described below.  The
+     <upstream_version> portion of the version number is mandatory.
+
+     The <upstream_version> may contain only alphanumerics[1] and the characters
+     ``.`` ``+`` ``-`` ``:`` ``~`` (full stop, plus, hyphen, colon, tilde) and
+     should start with a digit. If there is no <debian_revision> then hyphens
+     are not allowed; if there is no <epoch> then colons are not allowed.
+
+  <debian_revision>
+     This part of the version number specifies the version of the
+     Debian package based on the upstream version.  It may contain
+     only alphanumerics and the characters ``+`` ``.`` ``~`` (plus, full
+     stop, tilde) and is compared in the same way as the
+     <upstream_version> is.
+
+     It is optional; if it isn't present then the <upstream_version>
+     may not contain a hyphen.  This format represents the case where
+     a piece of software was written specifically to be turned into a
+     Debian package, and so there is only one "debianisation" of it
+     and therefore no revision indication is required.
+
+     It is conventional to restart the <debian_revision> at ``1`` each
+     time the <upstream_version> is increased.
+
+     The package management system will break the version number apart
+     at the last hyphen in the string (if there is one) to determine
+     the <upstream_version> and <debian_revision>.  The absence of a
+     <debian_revision> compares earlier than the presence of one (but
+     note that the <debian_revision> is the least significant part of
+     the version number).
+
+The <upstream_version> and <debian_revision> parts are compared by the
+package management system using the same algorithm:
+
+The strings are compared from left to right.
+
+First the initial part of each string consisting entirely of non-digit
+characters is determined.  These two parts (one of which may be empty)
+are compared lexically.  If a difference is found it is returned.  The
+lexical comparison is a comparison of ASCII values modified so that
+all the letters sort earlier than all the non-letters and so that a
+tilde sorts before anything, even the end of a part.  For example, the
+following parts are in sorted order from earliest to latest: ``~~``,
+``~~a``, ``~``, the empty part, ``a``.[2]
+
+Then the initial part of the remainder of each string which consists
+entirely of digit characters is determined.  The numerical values of
+these two parts are compared, and any difference found is returned as
+the result of the comparison.  For these purposes an empty string
+(which can only occur at the end of one or both version strings being
+compared) counts as zero.
+
+These two steps (comparing and removing initial non-digit strings and
+initial digit strings) are repeated until a difference is found or
+both strings are exhausted.
+
+Note that the purpose of epochs is to allow us to leave behind
+mistakes in version numbering, and to cope with situations where the
+version numbering scheme changes.  It is _not_ intended to cope with
+version numbers containing strings of letters which the package
+management system cannot interpret (such as ``ALPHA`` or ``pre-``), or
+with silly orderings (the author of this manual has heard of a package
+whose versions went ``1.1``, ``1.2``, ``1.3``, ``1``, ``2.1``, ``2.2``,
+``2`` and so forth).
+
+[1]  Alphanumerics are ``A-Za-z0-9`` only.
+
+[2]  One common use of ``~`` is for upstream pre-releases.  For example,
+     ``1.0~beta1~svn1245`` sorts earlier than ``1.0~beta1``, which sorts
+     earlier than ``1.0``.
 
 Semver version ordering formula.
 --------------------------------