Index: index.html
===================================================================
--- index.html	(revision 6228)
+++ index.html	(revision 6229)
@@ -43,9 +43,22 @@
 full control over an explicit layer: objects can be added or removed or
 changed any time. A virtual layer has no such flexibility: pcb-rnd computes
 its content from explicit layers and there's no way to change the result
-directly. A layer has a type and a location.
+directly. A layer is always part of a <i>layer group</i>.
+
+<h3> 2.3. layer groups </h3>
 <p>
-Layer types are:
+One or more explicit layers form a <i>layer group</i>. All pcb-rnd layers
+of a layer group will end up on the same physical layer. The visibility of
+layers in a layer group are toggled together. The main use of layer groups
+is to exploit that layers have different drawing color on screen: there
+can be a signal and a gnd layers with different color in the same layer group,
+on the same physical layer.
+<p>
+Since <i>layer groups</i> donate the physical layers, a board stack-up
+is built of layer groups. Substrates are layer groups without drawable
+layers in them.
+<p>
+Layer group types are:
 <ul>
 	<li> copper for signal layers
 	<li> silk for silkscreen layers
@@ -54,9 +67,7 @@
 	<li> (paste for solder paste - this is a virtual layer)
 </ul>
 <p>
-Note: pcb-rnd does not model substrate layers.
-<p>
-Layer locations are:
+Layer group locations are:
 <ul>
 	<li> top (used to be the "component side" in the age of thru-hole components)
 	<li> bottom (used to be the "solder side" in the age of thru-hole components)
@@ -64,9 +75,9 @@
 	<li> global (affects all physical layers, thus has no specific location)
 </ul>
 <p>
-Not all combination of type and location are supported, for example an internal
-silk layer does not make much sense. The table below lists whether a
-combination is supported or not.
+Not all combination of type and location are supported, for an outline layer
+is always global. The table below lists whether a combination is supported
+or not.
 <p>
 <table border=1>
 	<tr><th> &nbsp; <th> top <th> bottom <th> intern <th> global
@@ -77,15 +88,6 @@
 	<tr><th> paste  <td> yes <td> yes    <td> no     <td> no
 </table>
 
-<h3> 2.3. layer groups </h3>
-<p>
-One or more explicit layers form a <i>layer group</i>. All pcb-rnd layers
-of a layer group will end up on the same physical layer. The visibility of
-layers in a layer group are toggled together. The main use of layer groups
-is to exploit that layers have different drawing color on screen: there
-can be a signal and a gnd layers with different color in the same layer group,
-on the same physical layer.
-
 <h3> 2.4. Basic drawing objects </h3>
 <p>
 Pcb-rnd supports a small number of basic drawing objects, from which complex