<tr><td>Hexagonal polygon </td><td> Hexagonal prism </td></tr>
</table>
+When 2D elements are extruded, in addition to 3D elements segments are
+created on ribs of the result 3D mesh. Free edges of input 2D elements
+generate logically horizontal rib segments. Logically vertical rib
+segments are generated from nodes belonging to a sole input 2D element
+(a figure below illustrates this rule).
+
+\image html extru_rib_segs.png "Two triangles extruded: no vertical rib segments generated from nodes #2 and #3 as they are shared by both triangles"
+
<em>To use extrusion:</em>
<ol>
<li>From the \b Modification menu choose the \b Extrusion item or click
