diff --git a/Triangulation_3/doc/Triangulation_3/Triangulation_3.txt b/Triangulation_3/doc/Triangulation_3/Triangulation_3.txt
index aebb1563b38..63cd2fbd1ef 100644
--- a/Triangulation_3/doc/Triangulation_3/Triangulation_3.txt
+++ b/Triangulation_3/doc/Triangulation_3/Triangulation_3.txt
@@ -410,7 +410,7 @@ My_vertex(const Point&p, Cell_handle c) : Vb(p, c) {}
 The situation is exactly similar for cell base classes.
 Section \ref TDS3secdesign "Software Design" provides more detailed information.
 
-\section Triangulation_3ParallelAlgorithms Parallel algorithms
+\subsection Triangulation_3ParallelAlgorithms Parallel algorithms
 
 Parallel algorithms of `Delaunay_triangulation_3` and `Regular_triangulation_3` are enabled 
 if the `TriangulationDataStructure_3::Concurrency_tag` embeddded type is `Parallel_tag` and a