diff --git a/contributed_definitions/NXcsg.nxdl.xml b/base_classes/NXcsg.nxdl.xml
similarity index 68%
rename from contributed_definitions/NXcsg.nxdl.xml
rename to base_classes/NXcsg.nxdl.xml
index 941620273b..45ca439f09 100644
--- a/contributed_definitions/NXcsg.nxdl.xml
+++ b/base_classes/NXcsg.nxdl.xml
@@ -30,13 +30,16 @@
 	   xsi:schemaLocation="http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd"
 	   >
   <doc>
-    Constructive Solid Geometry base class, using :ref:`NXquadric` and :ref:`NXoff_geometry`</doc>
+    Constructive Solid Geometry (CSG) base class.
+    
+    Offers concepts for combining the definitions of leaf and
+    branching nodes of a CSG tree.</doc>
   <field
      name="operation">
     <doc>
       One of the standard construction solid geometry set operations,
-      or if the CSG is a pointer to the geometry provided by an
-      :ref:`NXquadric` or an :ref:`NXoff_geometry`. Takes values:
+      or statement IS_QUADRIC or IS_MESH if the CSG is a pointer to an
+      instance of a geometry class. Takes values:
     </doc>
     <enumeration>
       <item value="UNION" />
@@ -53,8 +56,8 @@
      minOccurs="0"
      maxOccurs="1">
    <doc>
-     The first operand of constructive solid geometry
-     operation. Compulsory if 'operation' is UNION, INTERSECTION,
+     The first operand of constructive solid geometry operation.
+     Compulsory if 'operation' is UNION, INTERSECTION,
      DIFFERENCE or COMPLEMENT.
    </doc>
   </group>
@@ -64,8 +67,8 @@
      minOccurs="0"
      maxOccurs="1">
     <doc>
-     The second operand of constructive solid geometry
-     operation. Compulsory if 'operation' is UNION, INTERSECTION or
+     The second operand of constructive solid geometry operation.
+     Compulsory if 'operation' is UNION, INTERSECTION or
      DIFFERENCE.
     </doc>
   </group>
@@ -75,11 +78,14 @@
      minOccurs="0"
      maxOccurs="1">
     <doc>
-      Path to a field that is either an :ref:`NXquadric` (if
-      'operation' = IS_QUADRIC) or an :ref:`NXoff_geometry` (if
-      'operation' = IS_MESH) that defines the surface making up the
-      constructive solid geometry component. Compulsory if 'operation'
-      is IS_QUADRIC or IS_MESH.
+      Path to a field that is an instance of one of several possible geometry
+      classes: Specifically, :ref:`NXquadric` if 'operation' is IS_QUADRIC,
+      :ref:`NXoff_geometry`, or other primitive based base classes
+      if 'operation' is IS_MESH.
+      
+      The instance defines the surface making up the constructive solid
+      geometry component. This field is compulsory if 'operation' is
+      IS_QUADRIC or IS_MESH.
     </doc>
   </field>
 </definition>
diff --git a/contributed_definitions/NXquadric.nxdl.xml b/base_classes/NXquadric.nxdl.xml
similarity index 98%
rename from contributed_definitions/NXquadric.nxdl.xml
rename to base_classes/NXquadric.nxdl.xml
index b7a07611c8..2408178f5a 100644
--- a/contributed_definitions/NXquadric.nxdl.xml
+++ b/base_classes/NXquadric.nxdl.xml
@@ -29,7 +29,7 @@
 	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	   xsi:schemaLocation="http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd"
 	   >
-  <doc>definition of a quadric surface.</doc>
+  <doc>Definition of a quadric surface.</doc>
   <field 
      name="parameters"
      type="NX_NUMBER"
diff --git a/contributed_definitions/NXsolid_geometry.nxdl.xml b/base_classes/NXsolid_geometry.nxdl.xml
similarity index 75%
rename from contributed_definitions/NXsolid_geometry.nxdl.xml
rename to base_classes/NXsolid_geometry.nxdl.xml
index fb5751f268..749ad0723f 100644
--- a/contributed_definitions/NXsolid_geometry.nxdl.xml
+++ b/base_classes/NXsolid_geometry.nxdl.xml
@@ -30,7 +30,13 @@
 	   xsi:schemaLocation="http://definition.nexusformat.org/nxdl/3.1 ../nxdl.xsd"
 	   >
   <doc>
-    the head node for constructively defined geometry
+    The head node for constructively defined geometry.
+    
+    * `S. Ghebi &lt;https://doi.org/10.1007/978-1-84800-115-2&gt;`_
+    * `L. H. Laidlaw &lt;https://doi.org/10.1145/15886.15904&gt;`_
+    
+    for an introduction into the topic of modeling shapes with constructive
+    solid geometry (CSG).
   </doc>
   <group
      type="NXquadric"
@@ -46,11 +52,13 @@
       Instances of :ref:`NXoff_geometry` making up elements of the geometry.
     </doc>
   </group>
+  <!--in the future allow also groups of other NXcg instances to make up the geometry-->
   <group
      type="NXcsg"
      minOccurs="0">
     <doc>
-      The geometries defined, made up of instances of :ref:`NXquadric` and :ref:`NXoff_geometry`.
+      The geometries defined, made up of e.g. instances of :ref:`NXquadric`, :ref:`NXoff_geometry`,
+      or instances of other base classes that define geometries.
     </doc>
   </group>
 </definition>