The Secondary Realms is a title used by Denizens of The House to describe all of the worlds, such as Earth, that are not a part of the universe that they inhabit. The worlds appearing in the story are:
Earth: Home to the main protagonist and a few of his aides, Arthur Penhaligon. It is here that the Morrow Days attempt to manipulate him into abdicating or giving up, by way of blackmail, money trouble and fear. The resplendent Morrow Days can only appear on Earth on their respective day, eg Lord Sunday can only manipulate events from 00:00 on Sunday Morning to 23:59 on Sunday Night.
Avraxyn: Home to the mould that Saturday turned in to Spirit-Eater, and where Lady Friday hides when she abdicates from her position, in order to escape the destruction of the Middle House and the demise of Arthur, The Piper and Saturday in her trap.
Unknown World (Drowned Wednesday): Where the damaged ship The Moth, carrying Arthur, washes up after escaping Feverfew and his ship. Arthur is later taken from here by Wednesday's Dawn.
Insectoid World: Arthur materialises here while escaping from the Incomparable Gardens during Lord Sunday . A war between tall insect-like warriors is taking place, who instantly turn on Arthur soon after he arrives.
Tuesday's World: Where Grim Tuesday hides his part of The Will of The Architect. It contains a huge rainforest/jungle and an ocean. This worldlet is inside a sun.
Feverfew's Worldlet: A world taken from the secondary realms and placed in the gut of the leviathan Drowned Wednesday. It is basically a simple island, based on designs from a world in the realms copied by Grim Tuesday.
The Void: An everlasting universe of nothing that consumes anything that touches it. It breaches into the House and the Realms at certain points during the story, mainly at places unprotected against the void. So far nothing has impinged on: The Lower House, The Far Reaches, the Border Sea, the Great Maze, the Upper House, and the Incomparable Gardens. At the end of Lord Sunday, when the Will is made whole, the Incomparable Gardens is the last piece of the House left, the other demesnes being destroyed by the void. It later destroys Sunday's Elysium and destroys the universe, something later turned back by the New Architect