The Middle Marches, also known as the Mittelmarch, are a fictional land found in the Von Bek series of novels written by Michael Moorcock. They are described as “the world in between heaven and ours”.
In my fiction, the Mittelmarche is an etheric world that stitches together the ‘normal’ parts of inhabited space. The technology of both the Portals and the Færie Doors create artificial spaces of the same nature.
