diff --git a/doc/html/property.html b/doc/html/property.html
index 281db7db..0d233d11 100644
--- a/doc/html/property.html
+++ b/doc/html/property.html
@@ -105,6 +105,10 @@ Attach a "property" (string key and value pair) to the edit cell
 	     are required to accommodate pins).  Any set-back applied by the
 	     "lef write -hide <value>" option will be applied to this obstruction
 	     box.
+	<DT> <B>flatten</B>
+	<DD> This property is used in conjunction with the "flatten -doproperty"
+	     command option and marks the cell for flattening.  Cells without
+	     this property defined will not be flattened.
 	<DT> <B>device</B>
 	<DD> This property declares that the cell is a device or contains a
 	     single device that is not a known extractable device defined in