diff --git a/docs/hazmat/backends/interfaces.rst b/docs/hazmat/backends/interfaces.rst
index 87fc6ab..4f15d5c 100644
--- a/docs/hazmat/backends/interfaces.rst
+++ b/docs/hazmat/backends/interfaces.rst
@@ -666,14 +666,23 @@
         :raises cryptography.exceptions.UnsupportedAlgorithm: This is raised
             when any backend specific criteria are not met.
 
-    .. method:: dh_parameters_supported(p, g)
+    .. method:: dh_parameters_supported(p, g, q=None)
 
         :param int p: The p value of the DH key.
 
         :param int g: The g value of the DH key.
 
-        :returns: ``True`` if the given values of ``p`` and ``g`` are supported
-            by this backend, otherwise ``False``.
+        :param int q: The q value of the DH key.
+
+        :returns: ``True`` if the given values of ``p``, ``g`` and ``q``
+            are supported by this backend, otherwise ``False``.
+
+    .. versionadded:: 1.8
+
+    .. method:: dh_x942_serialization_supported()
+
+        :returns: True if serialization of DH objects with
+            subgroup order (q) is supported by this backend.
 
 
 .. class:: ScryptBackend
diff --git a/docs/hazmat/primitives/asymmetric/dh.rst b/docs/hazmat/primitives/asymmetric/dh.rst
index 73e534e..759b265 100644
--- a/docs/hazmat/primitives/asymmetric/dh.rst
+++ b/docs/hazmat/primitives/asymmetric/dh.rst
@@ -223,7 +223,7 @@
 Numbers
 ~~~~~~~
 
-.. class:: DHParameterNumbers(p, g)
+.. class:: DHParameterNumbers(p, g, q=None)
 
     .. versionadded:: 0.8
 
@@ -239,7 +239,15 @@
 
         :type: int
 
-        The generator value. Must be 2 or 5.
+        The generator value. Must be 2 or 5 (Unless q is given).
+
+    .. attribute:: q
+
+        .. versionadded:: 1.8
+
+        :type: int
+
+        p subgroup order value.
 
 .. class:: DHPrivateNumbers(x, public_numbers)