A BaseLayer is a special kind of Layer.

• If a Layer is not a BaseLayer, it is an Overlay.
• BaseLayers are mutually exclusive -- OpenLayers can only ever show one BaseLayer at a time.
• To change between BaseLayers, the user uses the LayerSwitcher.
• An OpenLayers.Layer? is by default an Overlay (not a BaseLayer)
• For instructions on how to configure an existing Layer as a BaseLayer, see BaseLayersAndOverlays.
• For instructions on how to create your own BaseLayer, see CreateABaseLayer.