diff --git a/Documentation/md.txt b/Documentation/md.txt
index 4cf5972..80f1587 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -200,6 +200,11 @@
      This can be written only while the array is being assembled, not
      after it is started.
 
+  layout
+     The "layout" for the array for the particular level.  This is
+     simply a number that is interpretted differently by different
+     levels.  It can be written while assembling an array.
+
    new_dev
      This file can be written but not read.  The value written should
      be a block device number as major:minor.  e.g. 8:0