In The City Of Towers
The Planes consist of
The Material Planes, including Eberron, lie at the center. The Shadowfell and Feywild are reflections of the Material Planes that lie overlapped, surrounding and penetrating the Material Planes. The Elemental Chaos surrounds the Material Planes, leaking in at places where the elements are strong. The Astral Sea is the space between the planes; they can be navigated with the right tools allowing for passage to others.
There exists a phenomenon known as Portals, where two points in seperate planes overlap. Portals allow instantaneous transportation from one plane to another, though they often require a key of some kind. These keys can be anything from a physical object to a song or state of mind. Portals can wink in and out of existence without warning and some, particularly those that touch The Feywild, seem to move about without any apparent order.