One of the first things you need to have clear when working with OpenLayers is the base layer concept.
A base layer is a special kind of layer, which is always visible and determines some map properties such as projection and zoom levels.
A map can have more than one base layer but only one of them can be active at a time.
In addition, if you add more than one flagged base layer to the map, the first base layer added will be used as the map's active base layer.
This recipe will show you how to add layers to the map flagging them to be base layers. We are going to create a page with two maps side-by-side and every map will have a layer switcher control that allows you to control the map layers.