diff --git a/Utils/Zkey.mli b/Utils/Zkey.mli
index 3579634..4c636b0 100644
--- a/Utils/Zkey.mli
+++ b/Utils/Zkey.mli
@@ -20,6 +20,9 @@
             x1         y1         z1         x2         y2         z2              x3         y3         z3         x4         y4         z4
   ]}
 
+  The values of x,y,z should be positive and should not exceed 32767 for
+  [kind=3]. For all other kinds kinds the values should not exceed 1023.
+
 *)
 
 type t