diff --git a/STL_Extension/include/CGAL/tags.h b/STL_Extension/include/CGAL/tags.h
index c343ff411cb..d26dd44f4f4 100644
--- a/STL_Extension/include/CGAL/tags.h
+++ b/STL_Extension/include/CGAL/tags.h
@@ -54,6 +54,9 @@ struct Null_functor {
   typedef Null_tag second_argument_type; 
 };
 
+// For concurrency
+struct Sequential_tag {};
+struct Parallel_tag {};
 
 // A function that asserts a specific compile time tag
 // forcing its two arguments to have equal type.