diff --git a/quaternion.hpp b/quaternion.hpp
index 1d89a016..fc2a67da 100644
--- a/quaternion.hpp
+++ b/quaternion.hpp
@@ -46,6 +46,8 @@ namespace cruft {
 
         /// Constructs a quaternion that represents the rotation of a unit
         /// vector `a` to the unit vector `b`.
+        ///
+        /// The return value is guaranteed to be normalised.
         static quaternion from_to (vector<3,T> a, vector<3,T> b);
 
         /// Constructs a quaternion that represents the rotation needed to