diff --git a/src/pgraph/PGraph.py b/src/pgraph/PGraph.py
index 65f060ca..f92d00de 100644
--- a/src/pgraph/PGraph.py
+++ b/src/pgraph/PGraph.py
@@ -28,6 +28,7 @@ def __init__(
         metric: Callable[[NDArray], float] | str | None = None,
         heuristic: Callable[[NDArray], float] | str | None = None,
         verbose: bool = False,
+        dim: int | None = None,
     ):
         """
         Abstract base class for graphs
@@ -39,17 +40,25 @@ def __init__(
         :type heuristic: callable or str, optional
         :param verbose: print diagnostic information as vertices/edges are
             added, defaults to False
+        :param dim: required length of every vertex's ``coord``, defaults to
+            None (unconstrained -- vertices may have coordinates of any
+            length, or none at all)
+        :type dim: int, optional
+        :raises ValueError: ``dim`` is given but is not a positive integer
 
         This is the common base class of :class:`UGraph` and :class:`DGraph`
         and should not be instantiated directly.
 
-        :seealso: :class:`UGraph` :class:`DGraph`
+        :seealso: :class:`UGraph` :class:`DGraph` :meth:`add_vertex`
         """
+        if dim is not None and dim <= 0:
+            raise ValueError(f"dim must be a positive integer, got {dim!r}")
         # we use a list and a dict, the list respects the order of adding
         self._vertexlist: list[BaseVertex] = []
         self._vertexdict: dict[str, BaseVertex] = {}
         self._edgelist: set[Edge] = set()
         self._verbose = verbose
+        self._dim = dim
         self._ncomponents = 0
         self._connectivitychange = False
         if metric is None:
@@ -249,6 +258,8 @@ def add_vertex(
         :param name: name of vertex, defaults to "#i"
         :type name: str, optional
         :raises TypeError: ``coord`` is a ``BaseVertex`` of the wrong kind
+        :raises ValueError: the graph was constructed with ``dim``, and
+            ``coord`` is given but its length doesn't match
         :return: the added vertex
         :rtype: BaseVertex subclass
 
@@ -276,6 +287,18 @@ def add_vertex(
             >>> v2 = g.add_vertex(UVertex(coord=[1,1], name='v2'))
             >>> print(v2.name)
 
+        If the graph was constructed with a required ``dim`` (see
+        :meth:`_BaseGraph.__init__`), every embedded vertex must have a
+        coordinate of exactly that length -- adding one of the wrong length
+        raises ``ValueError``:
+
+        .. runblock:: pycon
+
+            >>> from pgraph import UGraph
+            >>> g = UGraph(dim=6)
+            >>> v1 = g.add_vertex(coord=[0, 0, 0, 0, 0, 0], name='pose1')
+            >>> print(v1)
+
         :seealso: :meth:`vertex_copy`
         """
         if isinstance(coord, self._vertex_cls):
@@ -288,6 +311,16 @@ def add_vertex(
         else:
             vertex = self._vertex_cls(coord, name=name)
 
+        if (
+            self._dim is not None
+            and vertex.coord is not None
+            and len(vertex.coord) != self._dim
+        ):
+            raise ValueError(
+                f"vertex coord has length {len(vertex.coord)}, "
+                f"but this graph requires dim={self._dim}"
+            )
+
         if name is None:
             name = vertex.name
         if name is None:
diff --git a/tests/test_graph.py b/tests/test_graph.py
index 94502b15..6e06f6fe 100644
--- a/tests/test_graph.py
+++ b/tests/test_graph.py
@@ -358,6 +358,29 @@ def test_add_vertex(self):
         self.assertTrue(v in g)
         self.assertTrue(v._graph, g)
 
+    def test_dim(self):
+
+        # unconstrained by default: any length, or no coord at all
+        g = UGraph()
+        g.add_vertex(coord=[1, 2])
+        g.add_vertex(coord=[1, 2, 3, 4])
+        g.add_vertex()
+        self.assertEqual(g.n, 3)
+
+        # dim enforces every embedded vertex has exactly that length
+        g = UGraph(dim=6)
+        g.add_vertex(coord=[0, 0, 0, 0, 0, 0], name='pose1')
+        g.add_vertex(name='untyped')  # no coord: not checked
+        with self.assertRaises(ValueError):
+            g.add_vertex(coord=[1, 2, 3], name='bad')
+        self.assertEqual(g.n, 2)
+
+        # dim must be a positive integer
+        with self.assertRaises(ValueError):
+            UGraph(dim=0)
+        with self.assertRaises(ValueError):
+            UGraph(dim=-1)
+
     def test_properties(self):
 
         g = UGraph()