--- a/data-cube/index.html	Fri Mar 08 13:24:31 2013 +0100
+++ b/data-cube/index.html	Fri Mar 08 13:46:34 2013 +0000
@@ -772,7 +772,15 @@
 </p>
   
 <p>The data cube vocabulary permits either representation approach to be used though they cannot be mixed
-  within the same data set.</p>
+  within the same data set. </p>
+
+<p>Both representation approaches require
+  that, for every point in the space of dimensions for which there is
+  an observation, then a value must be given for every measure. In the
+  case of multi-measure observations then each measure must be
+  present on each observation. In cubes which use a measure dimension
+  then there are sets of observations for each populated point in the
+  cube and within each of those sets there must be an observation giving each measure.</p>
   
 
 <section id="dsd-mm-obs">