diff --git a/models/dimers random walk.xml b/models/dimers random walk.xml
index c5696fd..16439a2 100644
--- a/models/dimers random walk.xml	
+++ b/models/dimers random walk.xml	
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <gem-graph-model version="0.2">
     <!-- Model identity informations -->
     <identity>
@@ -24,8 +24,15 @@
         </simulation>
 
         <space-param>
-            <dimension>1</dimension>
-            <size x="29" y="0" z="0"/>
+            <dimension>
+    <!-- loop_on_zero="true"  anything moved before zero reenters at the end of space-->
+    <!-- loop_on_zero="false" anything moved before zero is lost-->
+    <!-- loop_on_max="true"   anything moved beyond max reenters at the zero of space-->
+    <!-- loop_on_max="false"  anything moved beyond max is lost-->
+                <axis name="x" size="29"/>
+                <axis name="y" size="0"/>
+                <axis name="z" size="0"/>
+            </dimension>
             <!-- Site_multiplicity = number of  sites in a space unit. -->
             <!-- Each site points towards a neighbouring space unit. -->
             <!-- Several arrows can be stacked in the same site. -->
@@ -111,3 +118,4 @@
     </transitions>
 
 </gem-graph-model>
+
diff --git a/schemas/models_0.2.xmls b/schemas/models_0.2.xmls
index a4f255d..2c8a0a9 100644
--- a/schemas/models_0.2.xmls
+++ b/schemas/models_0.2.xmls
@@ -48,6 +48,16 @@
 
 <xs:element name="quote" substitutionGroup="g_ref"/>
 
+<xs:element name="axis">
+    <xs:complexType>
+        <xs:sequence>
+            <xs:element minOccurs="0" ref="quote"/>
+        </xs:sequence>
+        <xs:attribute name="name"  type="xs:string" use="required"/>
+        <xs:attribute name="size"  type="xs:integer" use="required"/>
+    </xs:complexType>
+</xs:element>
+
 <xs:element name="arrow">
     <xs:complexType>
         <xs:sequence>
@@ -137,13 +147,13 @@
                 <xs:complexType>
                 <xs:sequence>
 
-                    <xs:element name="dimension" type="xs:integer"/>
-
-                    <xs:element name="size">
-                        <xs:complexType>
-                            <xs:attributeGroup ref="xyz_site_nb"/>
+                    <xs:element name="dimension">
+                          <xs:complexType>
+                          <xs:sequence minOccurs="1" maxOccurs="unbounded">
+                                <xs:element ref="axis"/>
+                        </xs:sequence>
                         </xs:complexType>
-                    </xs:element>
+                  </xs:element>
 
                     <xs:element name="site_multiplicity" type="xs:integer"/>