diff --git a/docs/python-api.rst b/docs/python-api.rst
index 52d705f9de..287e81ec38 100644
--- a/docs/python-api.rst
+++ b/docs/python-api.rst
@@ -484,9 +484,9 @@ Table functions
 
 .. _sec_metadata_api:
 
-********
-Metadata
-********
+************
+Metadata API
+************
 
 The ``metadata`` module provides validation, encoding and decoding of metadata
 using a schema. See :ref:`sec_metadata`, :ref:`sec_metadata_api_overview` and
@@ -498,16 +498,20 @@ using a schema. See :ref:`sec_metadata`, :ref:`sec_metadata_api_overview` and
 
 .. autofunction:: register_metadata_codec
 
-.. _sec_stats_api:
+.. _sec_combinatorics_api:
 
-*************
-Combinatorics
-*************
-The following are generators for fully enumerating unique tree topologies.
-The position of a tree in the enumeration ``all_trees`` is given by
-:meth:`Tree.rank`. Inversely, a :class:`Tree` can be constructed from a
-position in the enumeration with :meth:`Tree.unrank`.
-See :ref:`sec_combinatorics` for details.
+*****************
+Combinatorics API
+*****************
+
+The combinatorics API deals with tree topologies, allowing them to be counted,
+listed and generated: see :ref:`sec_combinatorics` for a detailed description. Briefly,
+the position of a tree in the enumeration ``all_trees`` can be obtained using the tree's
+:meth:`~Tree.rank` method. Inversely, a :class:`Tree` can be constructed from a position
+in the enumeration with :meth:`Tree.unrank`. Generated trees are associated with a new
+tree sequence containing only that tree for the entire genome (i.e. with
+:attr:`~TreeSequence.num_trees` = 1 and a :attr:`~TreeSequence.sequence_length` equal to
+the :attr:`~Tree.span` of the tree).
 
 .. autofunction:: all_trees
 
diff --git a/python/tests/test_combinatorics.py b/python/tests/test_combinatorics.py
index b6a178e4d5..52d6ef0ff2 100644
--- a/python/tests/test_combinatorics.py
+++ b/python/tests/test_combinatorics.py
@@ -182,6 +182,16 @@ def test_generate_trees_roundtrip(self):
         for rank_tree, tsk_tree in zip(all_rank_trees, all_tsk_trees):
             assert rank_tree == RankTree.from_tsk_tree(tsk_tree)
 
+    def test_generate_treeseq_roundtrip(self):
+        n = 5
+        span = 9
+        all_rank_trees = RankTree.all_labelled_trees(n)
+        all_tsk_trees = tskit.all_trees(n, span=span)
+        for rank_tree, tsk_tree in zip(all_rank_trees, all_tsk_trees):
+            ts1 = tsk_tree.tree_sequence
+            ts2 = rank_tree.to_tsk_tree(span=span).tree_sequence
+            assert ts1.tables.equals(ts2.tables, ignore_provenance=True)
+
     def test_all_shapes_roundtrip(self):
         n = 5
         all_rank_tree_shapes = RankTree.all_unlabelled_trees(n)
@@ -389,6 +399,16 @@ def test_rank_errors_multiple_roots(self):
         with pytest.raises(ValueError):
             ts.first().rank()
 
+    def test_span(self):
+        n = 5
+        span = 8
+        # Create a start tree, with a single root
+        tsk_tree = tskit.Tree.unrank((0, 0), n, span=span)
+        assert tsk_tree.num_nodes == n + 1
+        assert tsk_tree.interval.left == 0
+        assert tsk_tree.interval.right == span
+        assert tsk_tree.tree_sequence.sequence_length == span
+
     def test_big_trees(self):
         n = 14
         shape = 22
diff --git a/python/tskit/combinatorics.py b/python/tskit/combinatorics.py
index 517da2e749..d3fe262a8e 100644
--- a/python/tskit/combinatorics.py
+++ b/python/tskit/combinatorics.py
@@ -255,7 +255,7 @@ def join_topologies(child_topologies):
         return RankTree(children).rank()
 
 
-def all_trees(num_leaves):
+def all_trees(num_leaves, span=1):
     """
     Generates all unique leaf-labelled trees with ``num_leaves``
     leaves. See :ref:`sec_combinatorics` on the details of this
@@ -264,25 +264,27 @@ def all_trees(num_leaves):
     chosen arbitrarily.
 
     :param int num_leaves: The number of leaves of the tree to generate.
+    :param float span: The genomic span of each returned tree.
     :rtype: tskit.Tree
     """
     for rank_tree in RankTree.all_labelled_trees(num_leaves):
-        yield rank_tree.to_tsk_tree()
+        yield rank_tree.to_tsk_tree(span=span)
 
 
-def all_tree_shapes(num_leaves):
+def all_tree_shapes(num_leaves, span=1):
     """
     Generates all unique shapes of trees with ``num_leaves`` leaves.
 
     :param int num_leaves: The number of leaves of the tree to generate.
+    :param float span: The genomic span of each returned tree.
     :rtype: tskit.Tree
     """
     for rank_tree in RankTree.all_unlabelled_trees(num_leaves):
         default_labelling = rank_tree.label_unrank(0)
-        yield default_labelling.to_tsk_tree()
+        yield default_labelling.to_tsk_tree(span=span)
 
 
-def all_tree_labellings(tree):
+def all_tree_labellings(tree, span=1):
     """
     Generates all unique labellings of the leaves of a
     :class:`tskit.Tree`. Leaves are labelled from the set
@@ -290,11 +292,12 @@ def all_tree_labellings(tree):
 
     :param tskit.Tree tree: The tree used to generate
         labelled trees of the same shape.
+    :param float span: The genomic span of each returned tree.
     :rtype: tskit.Tree
     """
     rank_tree = RankTree.from_tsk_tree(tree)
     for labelling in RankTree.all_labellings(rank_tree):
-        yield labelling.to_tsk_tree()
+        yield labelling.to_tsk_tree(span=span)
 
 
 class RankTree:
@@ -515,12 +518,19 @@ def from_tsk_tree(tree):
 
         return RankTree.from_tsk_tree_node(tree, tree.root)
 
-    def to_tsk_tree(self):
+    def to_tsk_tree(self, span=1):
+        """
+        Convert a ``RankTree`` into the only tree in a new tree sequence.
+
+        :param float span: The genomic span of the returned tree. The tree will cover
+            the interval :math:`[0, span)` and the :attr:`~Tree.tree_sequence` from which
+            the tree is taken will have its :attr:`~tskit.TreeSequence.sequence_length`
+            equal to ``span``.
+        """
         if set(self.labels) != set(range(self.num_leaves)):
             raise ValueError("Labels set must be equivalent to [0, num_leaves)")
 
-        seq_length = 1
-        tables = tskit.TableCollection(seq_length)
+        tables = tskit.TableCollection(span)
 
         def add_node(node):
             if node.is_leaf():
@@ -532,7 +542,7 @@ def add_node(node):
             max_child_time = max(tables.nodes.time[c] for c in child_ids)
             parent_id = tables.nodes.add_row(time=max_child_time + 1)
             for child_id in child_ids:
-                tables.edges.add_row(0, seq_length, parent_id, child_id)
+                tables.edges.add_row(0, span, parent_id, child_id)
 
             return parent_id
 
diff --git a/python/tskit/trees.py b/python/tskit/trees.py
index 7fd2abcf0d..31077334bd 100644
--- a/python/tskit/trees.py
+++ b/python/tskit/trees.py
@@ -847,7 +847,7 @@ def rank(self):
         return combinatorics.RankTree.from_tsk_tree(self).rank()
 
     @staticmethod
-    def unrank(rank, num_leaves):
+    def unrank(rank, num_leaves, *, span=1):
         """
         Reconstruct the tree of the given ``rank``
         (see :meth:`tskit.Tree.rank`) with ``num_leaves`` leaves.
@@ -859,11 +859,15 @@ def unrank(rank, num_leaves):
 
         :param tuple(int) rank: The rank of the tree to generate.
         :param int num_leaves: The number of leaves of the tree to generate.
+        :param float span: The genomic span of the returned tree. The tree will cover
+            the interval :math:`[0, \\text{span})` and the :attr:`~Tree.tree_sequence`
+            from which the tree is taken will have its
+            :attr:`~tskit.TreeSequence.sequence_length` equal to ``span``.
         :rtype: Tree
         :raises: ValueError: If the given rank is out of bounds for trees
             with ``num_leaves`` leaves.
         """
-        return combinatorics.RankTree.unrank(rank, num_leaves).to_tsk_tree()
+        return combinatorics.RankTree.unrank(rank, num_leaves).to_tsk_tree(span=span)
 
     def count_topologies(self, sample_sets=None):
         """