Time proceeds (“flows”) in one direction – before to after, past to future. It does not, however, take the most direct route, or indeed only one route. Just as there are forks in a river, choice and decision produce “parallel” timelines that also flow from past to future, though not necessarily the same future. In the end, however, time tends to convergence and separate timelines tend to recohere – that is, to become one timeline again. This is defined as a natural type of Uncreation, and is much like how rivers tend to converge in some larger body of water, whether a pond, lake, sea, or ocean.
As mentioned above, time is wont to converge. This rule is so prevalent that even deities follow it. Different timelines want to become as similar as possible, which explains why many worlds have very similar denizens. Elves, Orcs, Humans, Dwarves, Gnomes. Because of time's tendency to converge. In fact, true divergence – true difference in timelines – is extremely rare; most appearances of divergence are in fact only variation. Rue difference can actually produce bad effects on the structure of time. According to the Codex of Deeper Mysteries, time is structured by something like a “mold,” and true divergence produces “cracks” in this mold that can be reused, recreated, or otherwise.