diff --git a/py_tutorials/merge.py b/py_tutorials/merge.py
index 931d095a..abacd38e 100644
--- a/py_tutorials/merge.py
+++ b/py_tutorials/merge.py
@@ -3,28 +3,35 @@ from netgen.csg import *
 
 from ngsolve import ngsglobals
 ngsglobals.msg_level = 2
-
+# generate brick and mesh it
 geo1 = CSGeometry()
 geo1.Add (OrthoBrick( Pnt(0,0,0), Pnt(1,1,1) ))
 m1 = geo1.GenerateMesh (maxh=0.1)
 m1.Refine()
-m1.Refine()
-
 
+# generate sphere and mesh it
 geo2 = CSGeometry()
 geo2.Add (Sphere (Pnt(0.5,0.5,0.5), 0.1))
 m2 = geo2.GenerateMesh (maxh=0.05)
 m2.Refine()
 m2.Refine()
 
-print ("********************")
-print ("** start merging  **")
-print ("********************")
+print ("***************************")
+print ("** merging suface meshes **")
+print ("***************************")
 
+# create an empty mesh
 mesh = Mesh()
-fd_outside = mesh.Add (FaceDescriptor(surfnr=1,domin=1))
-fd_inside = mesh.Add (FaceDescriptor(surfnr=2,domin=2,domout=1))
 
+# a face-descriptor stores properties associated with a set of surface elements
+# bc .. boundary condition marker,
+# domin/domout .. domain-number in front/back of surface elements (0 = void),
+# surfnr .. number of the surface described by the face-descriptor
+
+fd_outside = mesh.Add (FaceDescriptor(bc=1,domin=1,surfnr=1))
+fd_inside = mesh.Add (FaceDescriptor(bc=2,domin=2,domout=1,surfnr=2))
+# copy all boundary points from first mesh to new mesh.
+# pmap1 maps point-numbers from old to new mesh
 
 pmap1 = { }
 for e in m1.Elements2D():
@@ -32,11 +39,17 @@ for e in m1.Elements2D():
         if (v not in pmap1):
             pmap1[v] = mesh.Add (m1[v])
 
+
+# copy surface elements from first mesh to new mesh
+# we have to map point-numbers:
+
 for e in m1.Elements2D():
     mesh.Add (Element2D (fd_outside, [pmap1[v] for v in e.vertices]))
 
 
 
+# same for the second mesh:
+
 pmap2 = { }
 for e in m2.Elements2D():
     for v in e.vertices: