How to denote timelines
A timeline can be denoted in the following format. n.xyz where n is the primary timeline and xyz is the specific branch.
The Spiral (n+1, n>0)
The spiral is where all of existence sits. The spiral starts at the Primordial Timeline (see bellow) where all is perfectly rational, as the spiral gets further away from the Primordial Timeline the universes become less and less rational. This is how we see the emergence of magic.
Each universe, or timeline, is denoted by n. A universe is specified as a section of the spiral between a big bang and the heat death that comes after.
Eventually the spiral will collapse in on itself destroying everything and bringing the birth of a new Primordial Timeline.
The Primordial Timeline (0)
All of time can be traced back to this one timeline. In the Primordial Timeline everything is perfectly rational. The Primordial Timeline is best represented as an ellipse, due to it connecting back to itself it never experiences heat death like the others. All life can ultimately be traced back to this timeline.
When a new multiverse is created only the Primordial Timeline exists until some anomaly happens that causes it to spiral out. In this case it is the time gods Anne and William.
Offshoots and Branches
From the spiral new branches are created. These branches branch off every time there could be a difference. For example if you were trying to decide between a long sword and a dagger a new timeline would be created for each decision, one where you pick the long sword and one where you pick the dagger.
These branches exist until their heat death when they merge back with the spiral at the heat death of the primary timeline. Once a universe hits its heat death nothing can exist in it except for a few exceptions.
Void
Everywhere outside of the spiral and Primordial Timeline is called the void. Entities that exist in the void have escaped the death of the spiral. However this comes at the trade off as best being described as a concept instead of a person.