- "It looked as if someone had cut a patch out of the air, about two metres from the edge of the road, a patch roughly square in shape and less than a metre across. If you were level with the patch so that it was edge-on, it was nearly invisible, and it was completely invisible from behind. You could only see it from the side nearest the road, and you couldn't see it easily even from there, because all you could see through it was exactly the same kind of thing that lay in front of it ont his side: a patch of grass lit by a street light."
- —Will Parry describing a window in Oxford[src]
Physical description and making
They were clean cuts that allowed the traveller to slip through into another world. They could travel into different terrains, underground, underwater and in the air. They could be made by the bearer of the subtle knife focusing on opening small loops in the fabric of the multiverse. When they pulled on the loop, a window was created.
Serafina Pekkala told Kirjava and Pantalaimon that the windows in the universe were leaking Dust. Xaphania said that she and the angels would close all windows in the universe, except the one in the land of the dead, because all of the Dust that was flowing out of these windows was destroying all the worlds. As well as that, every window opened by the subtle knife caused a Spectre to appear which is why the world of Cittàgazze was full of them, once every window was closed, Xaphania said that the angels would take care of the Spectres.