There are five main types of alternate history, I’ve found. Three are only technical: Alternate Presents, Alternate And Historical But Not AH and Secret History. Two are more firm: Alternate History As Setting, and Alternate History As Genre.
This is the kind of “Alternate History” that’s only so in the most nominal fashion. Almost none are ever sold as alternate history, and if it’s mentioned at all, it’s as a “ok, I guess this happened” when talking about the background. So if there’s a fictional city it takes place in, it’s an “alternate present”.
Really, almost all fiction falls into this category. The only fiction that doesn’t is historical fiction that contains no made-up characters and is explictly trying to reenact a historical event as quickly as possible.
Alternate And Historical But Not AH
This is a strange classification, and it mainly has to do with events in a piece of historical fiction that A: diverge from actual history, but, B: Doesn’t change the grand-scheme events, and C: Isn’t really sold or promoted as “alternate history.”
The prime example I have of this is, of all things, Kelly’s Heroes. Yes, there’s an ahistorical gold raid there in World War II, occurring long before the movie was made. But it’s not sold or considered alternate history, and doesn’t explore the ramification.
This is when there was a big divergence in the past but which didn’t cause an actual change in history as we know it until the moment of the story. The Casca series is “Secret History”-yes, an immortal Roman mentored Genghis Khan and killed Adolph Hitler, but the Mongols still achieved what they did and Hitler still died in a bunker in Berlin in 1945.
And speaking of that mustached man, my favorite “secret history” divergence comes from Jerry Ahern’s Survivalist series. That has many styles. It’s post-apocalyptic action, then it becomes science fiction, and then the “secret history” part kicks in with the plot point of Hitler’s dead body being kept in a New York facility. Did that change anything up until Book 1? Not really.
Those three are only really considered alternate history in the most broad view.
Alternate History As Setting
A lot of alternate history, and, more importantly, a lot of stories that aren’t sold as or even considered alternate history fall into this category. There is a big divergence, and it did have an effect.
And yet, it’s there as a setting, a backdrop, to a main story taking place in a clear genre. Lots of “Alternate History As Setting” pieces are sold as alternate history. Others basically aren’t-a stereotypical steampunk story, The Yiddish Policeman’s Union, and Hotline Miami (three very different pieces of fiction) are all “Alternate History As Setting”.
Alternate History As Genre
This is the top of the pyramid and is reserved for works where the alternate events themselves are front and center, driving the plot. It encompasses a lot of Harry Turtledove’s fiction and essentially every work written in the “pseudo-history book” style.
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As with every category, not everything fits exceptions and there’s bound to be countless exceptions/works of fiction that don’t clearly belong to any one or could reasonably be considered both. But I’ve found a lot of stuff can fit pretty neatly into them.