diff --git a/src/db/db/dbStreamLayers.cc b/src/db/db/dbStreamLayers.cc
index eb1260601..ac2cc9819 100644
--- a/src/db/db/dbStreamLayers.cc
+++ b/src/db/db/dbStreamLayers.cc
@@ -262,6 +262,43 @@ static std::string format_interval (ld_type l1, ld_type l2)
   }
 }
 
+static std::vector<std::pair<ld_type, ld_type> >
+extract_dt_intervals (const LayerMap::datatype_map &dt_map, int ll, bool &has_others)
+{
+  std::vector<std::pair<ld_type, ld_type> > res;
+
+  for (LayerMap::datatype_map::const_iterator d = dt_map.begin (); d != dt_map.end (); ) {
+
+    if (d->second.find (ll) != d->second.end ()) {
+
+      std::pair<ld_type, ld_type> dpi = d->first;
+
+      if (d->second.size () > 1) {
+        has_others = true;
+      }
+
+      LayerMap::datatype_map::const_iterator dd = d;
+      ++dd;
+      while (dd != dt_map.end () && dd->first.first == dpi.second && dd->second.find (ll) != dd->second.end ()) {
+        if (dd->second.size () > 1) {
+          has_others = true;
+        }
+        dpi.second = dd->first.second;
+        ++dd;
+      }
+
+      d = dd;
+
+      res.push_back (dpi);
+
+    } else {
+      ++d;
+    }
+
+  }
+  return res;
+}
+
 std::string
 LayerMap::mapping_str (unsigned int ll) const
 {
@@ -269,36 +306,37 @@ LayerMap::mapping_str (unsigned int ll) const
   bool f1 = true;
   bool is_mmap = false;
 
-  for (ld_map::const_iterator l = m_ld_map.begin (); l != m_ld_map.end (); ++l) {
+  for (ld_map::const_iterator l = m_ld_map.begin (); l != m_ld_map.end (); ) {
+
+    std::pair<ld_type, ld_type> lti = l->first;
+
+    std::vector<std::pair<ld_type, ld_type> > dti = extract_dt_intervals (l->second, ll, is_mmap);
+    ++l;
+    while (l != m_ld_map.end () && lti.second == l->first.first && extract_dt_intervals (l->second, ll, is_mmap) == dti) {
+      lti.second = l->first.second;
+      ++l;
+    }
 
     bool f2 = true;
-    for (datatype_map::const_iterator d = l->second.begin (); d != l->second.end (); ++d) {
+    for (std::vector<std::pair<ld_type, ld_type> >::const_iterator d = dti.begin (); d != dti.end (); ++d) {
 
-      if (d->second.find (ll) != d->second.end ()) {
+      //  create a string representation
+      if (!f2) {
+        s += ",";
+      } else {
 
-        if (d->second.size () > 1) {
-          is_mmap = true;
+        if (!f1) {
+          s += ";";
         }
+        f1 = false;
 
-        //  create a string representation
-        if (!f2) {
-          s += ",";
-        } else {
-
-          if (!f1) {
-            s += ";";
-          }
-          f1 = false;
-
-          s += format_interval (l->first.first, l->first.second);
-          s += "/";
-
-        }
-        f2 = false;
-
-        s += format_interval (d->first.first, d->first.second);
+        s += format_interval (lti.first, lti.second);
+        s += "/";
 
       }
+      f2 = false;
+
+      s += format_interval (d->first, d->second);
 
     }
     
diff --git a/src/db/unit_tests/dbStreamLayerTests.cc b/src/db/unit_tests/dbStreamLayerTests.cc
index 6b0b478b0..f031de1f3 100644
--- a/src/db/unit_tests/dbStreamLayerTests.cc
+++ b/src/db/unit_tests/dbStreamLayerTests.cc
@@ -38,7 +38,7 @@ TEST(1)
   EXPECT_EQ (lm.first_logical (db::LDPair (2, 0)).second, (unsigned int) 18);
   EXPECT_EQ (lm.first_logical (db::LDPair (0, 0)).first, false);
 
-  EXPECT_EQ (lm.mapping_str (18), "1/0;2-5/0");
+  EXPECT_EQ (lm.mapping_str (18), "1-5/0");
   EXPECT_EQ (lm.mapping_str (17), "1/5");
 
   lm.map (db::LDPair (2, 2), 18);
@@ -477,7 +477,7 @@ TEST(8)
   //  some
   lm.mmap_expr ("*/1-10", n++);
   EXPECT_EQ (lm.to_string (),
-    "layer_map('+*/0,1-10,11-*';'+*/1-10')"
+    "layer_map('+*/*';'+*/1-10')"
   );
   EXPECT_EQ (db::LayerMap::from_string_file_format (lm.to_string_file_format ()).to_string (), lm.to_string ());
 
@@ -485,7 +485,7 @@ TEST(8)
   lm.mmap_expr ("*/5,15", n++);
 
   EXPECT_EQ (lm.to_string (),
-    "layer_map('+*/0,1-4,5,6-10,11-14,15,16-*';'+*/1-4,5,6-10';'+*/5,15')"
+    "layer_map('+*/*';'+*/1-10';'+*/5,15')"
   );
   EXPECT_EQ (db::LayerMap::from_string_file_format (lm.to_string_file_format ()).to_string (), lm.to_string ());
 
@@ -496,8 +496,8 @@ TEST(8)
   EXPECT_EQ (set2string (lm.logical (db::LDPair (0, 10))), "0,1");
 
   //  NOTE: the leading "+" indicates that the listed layers may go somewhere else, so we can't plainly map them
-  EXPECT_EQ (lm.mapping_str (0), "+*/0,1-4,5,6-10,11-14,15,16-*");
-  EXPECT_EQ (lm.mapping_str (1), "+*/1-4,5,6-10");
+  EXPECT_EQ (lm.mapping_str (0), "+*/*");
+  EXPECT_EQ (lm.mapping_str (1), "+*/1-10");
   EXPECT_EQ (lm.mapping_str (2), "+*/5,15");
   EXPECT_EQ (lm.mapping_str (3), "");
 
@@ -516,7 +516,7 @@ TEST(8)
   //  some
   lm.mmap_expr ("1-10/*", n++);
   EXPECT_EQ (lm.to_string (),
-    "layer_map('+0/*;1-10/*;11-*/*';'+1-10/*')"
+    "layer_map('+*/*';'+1-10/*')"
   );
   EXPECT_EQ (db::LayerMap::from_string_file_format (lm.to_string_file_format ()).to_string (), lm.to_string ());
 
@@ -524,7 +524,7 @@ TEST(8)
   lm.mmap_expr ("5,15/*", n++);
 
   EXPECT_EQ (lm.to_string (),
-    "layer_map('+0/*;1-4/*;5/*;6-10/*;11-14/*;15/*;16-*/*';'+1-4/*;5/*;6-10/*';'+5/*;15/*')"
+    "layer_map('+*/*';'+1-10/*';'+5/*;15/*')"
   );
   EXPECT_EQ (db::LayerMap::from_string_file_format (lm.to_string_file_format ()).to_string (), lm.to_string ());
 
@@ -535,8 +535,8 @@ TEST(8)
   EXPECT_EQ (set2string (lm.logical (db::LDPair (10, 0))), "0,1");
 
   //  NOTE: the leading "+" indicates that the listed layers may go somewhere else, so we can't plainly map them
-  EXPECT_EQ (lm.mapping_str (0), "+0/*;1-4/*;5/*;6-10/*;11-14/*;15/*;16-*/*");
-  EXPECT_EQ (lm.mapping_str (1), "+1-4/*;5/*;6-10/*");
+  EXPECT_EQ (lm.mapping_str (0), "+*/*");
+  EXPECT_EQ (lm.mapping_str (1), "+1-10/*");
   EXPECT_EQ (lm.mapping_str (2), "+5/*;15/*");
   EXPECT_EQ (lm.mapping_str (3), "");
 
@@ -553,7 +553,7 @@ TEST(8)
   //  some
   lm.mmap_expr ("1-10/0-20", n++);
   EXPECT_EQ (lm.to_string (),
-    "layer_map('+0/*;1-10/0-20,21-*;11-*/*';'+1-10/0-20')"
+    "layer_map('+*/*';'+1-10/0-20')"
   );
   EXPECT_EQ (db::LayerMap::from_string_file_format (lm.to_string_file_format ()).to_string (), lm.to_string ());
 }
diff --git a/src/lay/lay/doc/about/layer_mapping.xml b/src/lay/lay/doc/about/layer_mapping.xml
index 81d6205db..2f3ea4c82 100644
--- a/src/lay/lay/doc/about/layer_mapping.xml
+++ b/src/lay/lay/doc/about/layer_mapping.xml
@@ -173,5 +173,33 @@
     </tr>
   </table>
 
+  <h2>Multi-mapping and unmapping</h2>
+
+  <p>
+  Layer mapping table support an advanced feature which is to duplicate input layers to
+  a number of output layers (1:n) mapping.
+  The feature is enabled by prepending a "+" to the mapping statement. The following
+  statement will first select layer 5/0 and additionally copy it to layer 1000/0:
+  </p>
+
+  <pre>5/0
++5/0: 1000/0
+</pre>
+
+  <p>
+  Unmapping removes the mapping for a specific layer or range. It is specified by prepending "-"
+  to the mapping expression. The following statement will map all datatypes of layer 5 to 0 except
+  for datatype 10 which is not considered.
+  </p>
+
+  <pre>5/*: 5/0
+-5/10
+</pre>
+
+  <p>
+  Unmapping cancels the mappings specified previously, so the order of statements becomes important
+  when using unmapping and multi-mapping.
+  </p>
+
 </doc>