diff --git a/Cartesian_kernel/include/CGAL/Cartesian/Circle_3.h b/Cartesian_kernel/include/CGAL/Cartesian/Circle_3.h
index 96dac082ad3..ba583f69e7d 100644
--- a/Cartesian_kernel/include/CGAL/Cartesian/Circle_3.h
+++ b/Cartesian_kernel/include/CGAL/Cartesian/Circle_3.h
@@ -30,6 +30,7 @@ class CircleC3 {
   typedef typename R_::Direction_3              Direction_3;
   typedef typename R_::FT                       FT;
 
+  //using a boost::tuple because std::pair and tuple cannot work with incomplete types.
   typedef boost::tuple<Sphere_3, Plane_3> Rep;