Point of Divergence

Other tropes are widespread. Some are inevitable. In this genre, though, one trope is mandatory, enough to merit bold italics: the Point of Divergence.
A Point of Divergence (hereafter PoD) is what it sounds like: the first point at which our timeline diverges meaningfully from the author's, and the author starts to track the changes. Without a Point of Divergence, what's being written isn't alternate history yet at all.
The emphasis is on meaningful here. If any OTL Unknowns feature in the cast, there was obviously a divergence when they were born. (If not when their parents were born, etc.) It obviously diverged a little bit more over the course of their lives. None of that matters. Until the Point of Divergence, everything your audience can reasonably be expected to know about history is assumed to be true. After it, there are no guarantees, and it's your job to keep them informed.
Category:AH Tropes 101