diff --git a/hgext/rebase.py b/hgext/rebase.py
--- a/hgext/rebase.py
+++ b/hgext/rebase.py
@@ -28,8 +28,40 @@ def rebase(ui, repo, **opts):
     """move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of
-    history onto another. This can be useful for linearizing local
-    changes relative to a master development tree.
+    history (the source) onto another (the destination). This can be
+    useful for linearizing local changes relative to a master
+    development tree.
+
+    If you don't specify a destination changeset (``-d/--dest``),
+    rebase uses the tipmost head of the current named branch as the
+    destination. (The destination changeset is not modified by
+    rebasing, but new changesets are added as its descendants.)
+
+    You can specify which changesets to rebase in two ways: as a
+    \"source\" changeset or as a \"base\" changeset. Both are
+    shorthand for a topologically related set of changesets (the
+    \"source branch\"). If you specify source (``-s/--source``),
+    rebase will rebase that changeset and all of its descendants onto
+    dest. If you specify base (``-b/--base``), rebase will select
+    ancestors of base back to but not including the common ancestor
+    with dest. Thus, ``-b`` is less precise but more convenient than
+    ``-s``: you can specify any changeset in the source branch, and
+    rebase will select the whole branch. If you specify neither ``-s``
+    nor ``-b``, rebase uses the parent of the working directory as the
+    base.
+
+    By default, rebase recreates the changesets in the source branch
+    as descendants of dest and then destroys the originals. Use
+    ``--keep`` to preserve the original source changesets. Some
+    changesets in the source branch (e.g. merges from the destination
+    branch) may be dropped if they no longer contribute any change.
+
+    One result of the rules for selecting the destination changeset
+    and source branch is that, unlike ``merge``, rebase will do
+    nothing if you are at the latest (tipmost) head of a named branch
+    with two heads. You need to explicitly specify source and/or
+    destination (or ``update`` to the other head, if it's the head of
+    the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
@@ -490,9 +522,10 @@ cmdtable = {
 "rebase":
         (rebase,
         [
-        ('s', 'source', '', _('rebase from a given revision')),
-        ('b', 'base', '', _('rebase from the base of a given revision')),
-        ('d', 'dest', '', _('rebase onto a given revision')),
+        ('s', 'source', '', _('rebase from the specified changeset')),
+        ('b', 'base', '', _('rebase from the base of the specified changeset '
+                            '(up to greatest common ancestor of base and dest)')),
+        ('d', 'dest', '', _('rebase onto the specified changeset')),
         ('', 'collapse', False, _('collapse the rebased changesets')),
         ('', 'keep', False, _('keep original changesets')),
         ('', 'keepbranches', False, _('keep original branch names')),
@@ -501,6 +534,6 @@ cmdtable = {
         ('c', 'continue', False, _('continue an interrupted rebase')),
         ('a', 'abort', False, _('abort an interrupted rebase'))] +
          templateopts,
-        _('hg rebase [-s REV | -b REV] [-d REV] [--collapse] [--detach] '
-                        '[--keep] [--keepbranches] | [-c] | [-a]')),
+        _('hg rebase [-s REV | -b REV] [-d REV] [options]\n'
+          'hg rebase {-a|-c}'))
 }
diff --git a/tests/test-rebase-parameters.out b/tests/test-rebase-parameters.out
--- a/tests/test-rebase-parameters.out
+++ b/tests/test-rebase-parameters.out
@@ -2,22 +2,52 @@
 
 % Use continue and abort
 hg rebase: cannot use both abort and continue
-hg rebase [-s REV | -b REV] [-d REV] [--collapse] [--detach] [--keep] [--keepbranches] | [-c] | [-a]
+hg rebase [-s REV | -b REV] [-d REV] [options]
+hg rebase {-a|-c}
 
 move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of history
-    onto another. This can be useful for linearizing local changes relative to
-    a master development tree.
+    (the source) onto another (the destination). This can be useful for
+    linearizing local changes relative to a master development tree.
+
+    If you don't specify a destination changeset ("-d/--dest"), rebase uses
+    the tipmost head of the current named branch as the destination. (The
+    destination changeset is not modified by rebasing, but new changesets are
+    added as its descendants.)
+
+    You can specify which changesets to rebase in two ways: as a "source"
+    changeset or as a "base" changeset. Both are shorthand for a topologically
+    related set of changesets (the "source branch"). If you specify source
+    ("-s/--source"), rebase will rebase that changeset and all of its
+    descendants onto dest. If you specify base ("-b/--base"), rebase will
+    select ancestors of base back to but not including the common ancestor
+    with dest. Thus, "-b" is less precise but more convenient than "-s": you
+    can specify any changeset in the source branch, and rebase will select the
+    whole branch. If you specify neither "-s" nor "-b", rebase uses the parent
+    of the working directory as the base.
+
+    By default, rebase recreates the changesets in the source branch as
+    descendants of dest and then destroys the originals. Use "--keep" to
+    preserve the original source changesets. Some changesets in the source
+    branch (e.g. merges from the destination branch) may be dropped if they no
+    longer contribute any change.
+
+    One result of the rules for selecting the destination changeset and source
+    branch is that, unlike "merge", rebase will do nothing if you are at the
+    latest (tipmost) head of a named branch with two heads. You need to
+    explicitly specify source and/or destination (or "update" to the other
+    head, if it's the head of the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
 options:
 
- -s --source        rebase from a given revision
- -b --base          rebase from the base of a given revision
- -d --dest          rebase onto a given revision
+ -s --source        rebase from the specified changeset
+ -b --base          rebase from the base of the specified changeset (up to
+                    greatest common ancestor of base and dest)
+ -d --dest          rebase onto the specified changeset
     --collapse      collapse the rebased changesets
     --keep          keep original changesets
     --keepbranches  keep original branch names
@@ -31,22 +61,52 @@ use "hg -v help rebase" to show global o
 
 % Use continue and collapse
 hg rebase: cannot use collapse with continue or abort
-hg rebase [-s REV | -b REV] [-d REV] [--collapse] [--detach] [--keep] [--keepbranches] | [-c] | [-a]
+hg rebase [-s REV | -b REV] [-d REV] [options]
+hg rebase {-a|-c}
 
 move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of history
-    onto another. This can be useful for linearizing local changes relative to
-    a master development tree.
+    (the source) onto another (the destination). This can be useful for
+    linearizing local changes relative to a master development tree.
+
+    If you don't specify a destination changeset ("-d/--dest"), rebase uses
+    the tipmost head of the current named branch as the destination. (The
+    destination changeset is not modified by rebasing, but new changesets are
+    added as its descendants.)
+
+    You can specify which changesets to rebase in two ways: as a "source"
+    changeset or as a "base" changeset. Both are shorthand for a topologically
+    related set of changesets (the "source branch"). If you specify source
+    ("-s/--source"), rebase will rebase that changeset and all of its
+    descendants onto dest. If you specify base ("-b/--base"), rebase will
+    select ancestors of base back to but not including the common ancestor
+    with dest. Thus, "-b" is less precise but more convenient than "-s": you
+    can specify any changeset in the source branch, and rebase will select the
+    whole branch. If you specify neither "-s" nor "-b", rebase uses the parent
+    of the working directory as the base.
+
+    By default, rebase recreates the changesets in the source branch as
+    descendants of dest and then destroys the originals. Use "--keep" to
+    preserve the original source changesets. Some changesets in the source
+    branch (e.g. merges from the destination branch) may be dropped if they no
+    longer contribute any change.
+
+    One result of the rules for selecting the destination changeset and source
+    branch is that, unlike "merge", rebase will do nothing if you are at the
+    latest (tipmost) head of a named branch with two heads. You need to
+    explicitly specify source and/or destination (or "update" to the other
+    head, if it's the head of the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
 options:
 
- -s --source        rebase from a given revision
- -b --base          rebase from the base of a given revision
- -d --dest          rebase onto a given revision
+ -s --source        rebase from the specified changeset
+ -b --base          rebase from the base of the specified changeset (up to
+                    greatest common ancestor of base and dest)
+ -d --dest          rebase onto the specified changeset
     --collapse      collapse the rebased changesets
     --keep          keep original changesets
     --keepbranches  keep original branch names
@@ -60,22 +120,52 @@ use "hg -v help rebase" to show global o
 
 % Use continue/abort and dest/source
 hg rebase: abort and continue do not allow specifying revisions
-hg rebase [-s REV | -b REV] [-d REV] [--collapse] [--detach] [--keep] [--keepbranches] | [-c] | [-a]
+hg rebase [-s REV | -b REV] [-d REV] [options]
+hg rebase {-a|-c}
 
 move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of history
-    onto another. This can be useful for linearizing local changes relative to
-    a master development tree.
+    (the source) onto another (the destination). This can be useful for
+    linearizing local changes relative to a master development tree.
+
+    If you don't specify a destination changeset ("-d/--dest"), rebase uses
+    the tipmost head of the current named branch as the destination. (The
+    destination changeset is not modified by rebasing, but new changesets are
+    added as its descendants.)
+
+    You can specify which changesets to rebase in two ways: as a "source"
+    changeset or as a "base" changeset. Both are shorthand for a topologically
+    related set of changesets (the "source branch"). If you specify source
+    ("-s/--source"), rebase will rebase that changeset and all of its
+    descendants onto dest. If you specify base ("-b/--base"), rebase will
+    select ancestors of base back to but not including the common ancestor
+    with dest. Thus, "-b" is less precise but more convenient than "-s": you
+    can specify any changeset in the source branch, and rebase will select the
+    whole branch. If you specify neither "-s" nor "-b", rebase uses the parent
+    of the working directory as the base.
+
+    By default, rebase recreates the changesets in the source branch as
+    descendants of dest and then destroys the originals. Use "--keep" to
+    preserve the original source changesets. Some changesets in the source
+    branch (e.g. merges from the destination branch) may be dropped if they no
+    longer contribute any change.
+
+    One result of the rules for selecting the destination changeset and source
+    branch is that, unlike "merge", rebase will do nothing if you are at the
+    latest (tipmost) head of a named branch with two heads. You need to
+    explicitly specify source and/or destination (or "update" to the other
+    head, if it's the head of the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
 options:
 
- -s --source        rebase from a given revision
- -b --base          rebase from the base of a given revision
- -d --dest          rebase onto a given revision
+ -s --source        rebase from the specified changeset
+ -b --base          rebase from the base of the specified changeset (up to
+                    greatest common ancestor of base and dest)
+ -d --dest          rebase onto the specified changeset
     --collapse      collapse the rebased changesets
     --keep          keep original changesets
     --keepbranches  keep original branch names
@@ -89,22 +179,52 @@ use "hg -v help rebase" to show global o
 
 % Use source and base
 hg rebase: cannot specify both a revision and a base
-hg rebase [-s REV | -b REV] [-d REV] [--collapse] [--detach] [--keep] [--keepbranches] | [-c] | [-a]
+hg rebase [-s REV | -b REV] [-d REV] [options]
+hg rebase {-a|-c}
 
 move changeset (and descendants) to a different branch
 
     Rebase uses repeated merging to graft changesets from one part of history
-    onto another. This can be useful for linearizing local changes relative to
-    a master development tree.
+    (the source) onto another (the destination). This can be useful for
+    linearizing local changes relative to a master development tree.
+
+    If you don't specify a destination changeset ("-d/--dest"), rebase uses
+    the tipmost head of the current named branch as the destination. (The
+    destination changeset is not modified by rebasing, but new changesets are
+    added as its descendants.)
+
+    You can specify which changesets to rebase in two ways: as a "source"
+    changeset or as a "base" changeset. Both are shorthand for a topologically
+    related set of changesets (the "source branch"). If you specify source
+    ("-s/--source"), rebase will rebase that changeset and all of its
+    descendants onto dest. If you specify base ("-b/--base"), rebase will
+    select ancestors of base back to but not including the common ancestor
+    with dest. Thus, "-b" is less precise but more convenient than "-s": you
+    can specify any changeset in the source branch, and rebase will select the
+    whole branch. If you specify neither "-s" nor "-b", rebase uses the parent
+    of the working directory as the base.
+
+    By default, rebase recreates the changesets in the source branch as
+    descendants of dest and then destroys the originals. Use "--keep" to
+    preserve the original source changesets. Some changesets in the source
+    branch (e.g. merges from the destination branch) may be dropped if they no
+    longer contribute any change.
+
+    One result of the rules for selecting the destination changeset and source
+    branch is that, unlike "merge", rebase will do nothing if you are at the
+    latest (tipmost) head of a named branch with two heads. You need to
+    explicitly specify source and/or destination (or "update" to the other
+    head, if it's the head of the intended source branch).
 
     If a rebase is interrupted to manually resolve a merge, it can be
     continued with --continue/-c or aborted with --abort/-a.
 
 options:
 
- -s --source        rebase from a given revision
- -b --base          rebase from the base of a given revision
- -d --dest          rebase onto a given revision
+ -s --source        rebase from the specified changeset
+ -b --base          rebase from the base of the specified changeset (up to
+                    greatest common ancestor of base and dest)
+ -d --dest          rebase onto the specified changeset
     --collapse      collapse the rebased changesets
     --keep          keep original changesets
     --keepbranches  keep original branch names