I could have sworn that someone said that that at one time you could only cut out of a world and into Citagazze and that to travel between worlds you had to travel via Cittagazze. Anyone remember this or am I nutty as peanut butter?
It's not that you could
only cut into Cittagazze, just that the bearer would almost always cut into his own world when trying to leave a world.
When Will first got the knife, he would travel only between his world and Cittagazze. Then he realized something: the particles that the knife caught on had individual qualities to them. Each one felt a little different, and each one lead to a different world.
Before that realization, Will was automatically cutting into the first world he felt with the knife. After a while he could tell which was which. Perhaps the bearers before him never figured this out, and would cut into the first world they felt.
Scratch that... If there were windows into multiple worlds, they must have been able to notice the difference.
Or perhaps one bearer would feel a different world from another, so over time there were windows to many worlds, though each individual bearer would only cut into one in particular. Or it happened that the first world they felt was different in different places.
Yes, I realize that I just debated with myself there. So what.