"Double Sided" is the default setting for new meshes in Blender.  By turning off the "Double Sided" button for a mesh the triangle count for that mesh will be automatically cut in half.

If any Blender faces or meshes are set to "Double Sided", the exporter will automatically generate a backwards facing version of the faces.  On OpenGL and Direct3D graphics hardware, triangles are inherently single sided.

The face normals of the new back faces will point in the opposite direction as their corresponding front face.

The vertex normals of the new back faces will point in the same direction as the corresponding front face.  This means that the front and back faces are lit the same way, as if the light were coming through the polygon and out the other side.  This may not always be what is desired, therefore the direction of vertex normals for generated back faces may become a toggleable option in future versions of the Blender DTS exporter.  For now, if you need the vertex normals to face the opposite direction for back faces, it is recommended that you generate the backwards facing polygons by hand instead of using the double sided setting.

Setting meshes or faces as double sided

1. Select the desired mesh in object mode
2. Toggle the double sided setting on the Mesh panel of the editing buttons (F9):
   
   The selected mesh is now double sided or single sided, depending on the state of the double sided button

The same button also works in edit mode for setting individual faces as single or double sided in versions of Blender prior to 2.45.  In Blender 2.45 and above, the ability to set individual faces as single or double sided appears to have been removed.

See also:

• Vertex Normals
• Face Normals