diff --git a/vector.hpp b/vector.hpp
index ff4833d8..ed224485 100644
--- a/vector.hpp
+++ b/vector.hpp
@@ -131,6 +131,8 @@ namespace cruft {
 
 
     // polar/cartesian conversions; assumes (mag, angle) form.
+    //
+    // The angle is specified in radians.
     template <typename T> vector<2,T> polar_to_cartesian (vector<2,T>);
     template <typename T> vector<2,T> cartesian_to_polar (vector<2,T>);