diff --git a/src/pgraph/PGraph.py b/src/pgraph/PGraph.py
index cfc7255d..d44b815a 100644
--- a/src/pgraph/PGraph.py
+++ b/src/pgraph/PGraph.py
@@ -25,6 +25,7 @@ class BaseGraph(ABC):
 
     def __init__(
         self,
+        *,
         metric: Callable[[NDArray], float] | str | None = None,
         heuristic: Callable[[NDArray], float] | str | None = None,
         verbose: bool = False,
@@ -60,6 +61,14 @@ def __init__(
         This is the common base class of :class:`UGraph` and :class:`DGraph`
         and should not be instantiated directly.
 
+        All arguments are keyword-only, so ``UGraph(dim=6)`` is correct and
+        ``UGraph(6)`` raises :exc:`TypeError`.
+
+        .. versionchanged:: 1.0
+            Arguments are keyword-only.  In 0.6.5 the first positional
+            argument was accepted and ignored, so ``UGraph(6)`` silently did
+            nothing.
+
         ``vertex_class`` and ``edge_class`` let user subclasses be built by the
         graph, so they never need to be constructed by hand::
 
diff --git a/tests/test_constructor_keywords.py b/tests/test_constructor_keywords.py
new file mode 100644
index 00000000..c8d042b3
--- /dev/null
+++ b/tests/test_constructor_keywords.py
@@ -0,0 +1,40 @@
+"""Graph constructor arguments are keyword-only.
+
+In 0.6.5 the first positional argument was accepted and ignored, so
+``UGraph(6)`` looked as though it set a dimension but did nothing.
+"""
+
+import unittest
+
+from pgraph import DGraph, UGraph
+
+
+class _Keywords:
+    cls: type
+
+    def test_positional_rejected(self):
+        for arg in (6, "L2", None, True):
+            with self.subTest(arg=arg):
+                with self.assertRaises(TypeError):
+                    self.cls(arg)
+
+    def test_keywords_accepted(self):
+        g = self.cls(dim=6, metric="L1", heuristic="L2", verbose=False)
+        self.assertEqual(g.n, 0)
+        with self.assertRaises(ValueError):
+            g.add_vertex([1, 2])
+
+    def test_no_arguments(self):
+        self.assertEqual(self.cls().n, 0)
+
+
+class TestUGraphKeywords(_Keywords, unittest.TestCase):
+    cls = UGraph
+
+
+class TestDGraphKeywords(_Keywords, unittest.TestCase):
+    cls = DGraph
+
+
+if __name__ == "__main__":
+    unittest.main()