Historical fiction. Historical fantasy. Slipstream. Alternate history. Alternate universe. Where do the divisions lie? The truth is, it’s hard to find a black-and-white answer, and if you consider the terms as marketing categories rather than genre descriptions, things get even blurrier.
The thing is, whenever you add ‘fiction’ to your ‘history’, you’ve already crossed a line. You are introducing characters that didn’t really exist in that time period. Even if you steal names and dates from tombstones, you’re going to be attributing thoughts, emotions, and at least some actions to those characters that come from nowhere but your own imagination. But how much liberty can and should the author take? There’s a vast range of opinion on that topic. It’s at least a good idea to know the state of technology in the era you’re writing in. For example, purple-dyed cloth was difficult and expensive to achieve in medieval times, so if you dress your peasant lass in a purple dress with gilt trim, you will probably lose half your audience in one unfortunate clothing choice. And please, for gods’ sake, don’t put your lady in a corset if you don’t know for absolute certain that women corseted in that period!
But there’s a range of readers and therefore a range of what readers will accept in the guise of historical fiction. I once heard a historian go on a fifteen-minute rant because a certain very respected author of literary historical fiction made up a fort rather than using one that was known to exist. Ironically, I suspect he did so to try to preemptively avoid the historian hate mail he would get if he tried to put fictional characters in charge of a known historical fort.
Historical fantasy has a little more leeway. After all, we’re fairly certain that King Richard did not have an elven lord in his retinue, and so if we sign off on that, we may as well accept that little extra nation that the author nudged into Europe. (The Hound and the Falcon trilogy, and BTW author Judith Tarr has a degree in history and, other than the magic she slips in and the Richard who is not entirely a heartless monster, her actual historical details are probably more accurate than many history books.) My gaslamp fantasy/steampunk Victorian detective novel is a little bit further afield, as I have werewolves openly running through the streets of London, as well as alchemy that works. Also, my clockwork and steam technology is a bit more advanced than it actually had been in period.
A lot of books that are really slipstream get marketed under historical fantasy simply because slipstream is hard to define and no one knows what it is anyway. For example, Amazon put my novel Brother to the Wolf in the historical fantasy category. I’m not about to argue the placement, since I believe many readers of historical fantasy would enjoy it. Certainly, anyone familiar with the political and social situation of post-Norman Conquest England would recognize the background stage on which my characters move. History, however, stubbornly failed to yield to me a crown prince and a regent that exactly fit my plot. I have enough respect for history (and enough fear of historians) that I wasn’t going to claim to be dealing with Normans versus Saxons and yet invent characters and events that never existed in real history. (You can get away with it for peasants, yeoman, foresters, and the like, because their lives aren’t documented like that of the royals and their administrators.) By strict genre classification, Brother is slipstream, or maybe some genre classification of its own. (Pagan spiritual, historically-influenced fantasy just doesn’t roll off the tongue, somehow.)
Alternate history happens when doctoral candidates argue over their beers about what the world would look like if Hitler won or the South rose again, and someone decides that writing a novel is more fun than writing a thesis. (I’m being flip, of course, but you get the idea.) Alternate universes can be historical or contemporary. They’re what happen when a fantasy writer tries to set a novel in the real world, but things get out of hand. Think of it as a continuum between slipstream through alternate-universe fantasy to historical fantasy, although you could argue all night about where one ends and another begins.
Actually, I’d be happy to argue the topic all night, if someone else buys my whiskey. But since I’m not seeing any immediate takers, I will sign off here and let you discuss it amongst yourselves.