diff --git a/src/lay/lay/doc/manual/lvs_device_extractors.xml b/src/lay/lay/doc/manual/lvs_device_extractors.xml
index b6fdbe30a..2c43ca6d3 100644
--- a/src/lay/lay/doc/manual/lvs_device_extractors.xml
+++ b/src/lay/lay/doc/manual/lvs_device_extractors.xml
@@ -3,9 +3,9 @@
 
 <doc>
 
-  <title>LVS Devices Extractors</title>
+  <title>LVS Device Extractors</title>
   <keyword name="LVS"/>
-  <keyword name="LVS Devices Extractors"/>
+  <keyword name="LVS Device Extractors"/>
 
   <h2-index/>
 
@@ -345,5 +345,32 @@ extract_devices(bjt3(model_name), { "C" => collector, "B" => base, "E" => emitte
   <img src="/manual/bjtlat_ex_ts.png"/>
   </p>
 
+  <h2>Device extractors and device classes</h2>
+
+  <p>
+  "extract_devices" will return the <class_doc href="DeviceClass">DeviceClass</class_doc> object of the devices generated.
+  This object can be useful to apply some basic modifications.
+  The most important of them is enabling or disabling certain parameters.
+  </p>
+
+  <p>
+  Most device extractors extract more parameters than they give you by default.
+  For example, the resitor extractor will not just extract the resistance, but also the length (L)
+  and width (W) of the resistor stripe and its area (A) and perimeter (P).
+  By default these additional parameters are declared "secondary" - i.e. they will not
+  show up in the netlist browser, they will not participate in the device compare and
+  will not be netlisted.
+  </p>
+
+  <p>
+  Parameters can be declared primary ones by using "enable_parameter" on the device class.
+  Hence it is possible to enable "W" and "L" on a resistor type using the following code:
+  </p>
+
+  <pre>dc = extract_devices(resistor("RES", 1), ...)
+dc.enable_parameter("W", true)
+dc.enable_parameter("L", true)
+</pre>
+
 </doc>