The past is another country — but an alternate history is a whole new world.

The best alternate histories can make you see the real history of our world in a whole new way, and make you realize that events that seem like they were inevitable... may not have been.

Failing to bring it up to the present.

If you don't bring your alternate history up to the reader's present, then you leave out half the fun

"Philip Roth, who had never read and therefore didn't know he was writing alternate history, committed this sin of omission in his otherwise excellent The Plot Against America,"

Not recognizing that some historical developments were probably inevitable.

So if you wanted to change the Age of Exploration, says Stirling, you'd need a fairly major change, much earlier in history — not just a major setback for Christopher Columbus.

Ignoring historical factors that were important at the time, even if they aren't important to your story.

A big part of writing alternate history, is making judgments about which historical points to pursue, and which to "let fall by the wayside," says Boneshaker author Cherie Priest.

Priest says she still believes in picking and choosing which details to focus on, but has also tried to "take a more holistic approach to my alternate timeline building - giving at least a nod to the things that were contemporarily important, even if they aren't immediately critical to my fiction."

Not accounting for even the most obvious ripples from one big change

You can't account for all of the ripples from one point of departure (POD), because those ripples will have ripples, and so on

But you can, and should, pay attention to things that almost happened in real history — because they might well have happened, if things were different.

Concentrating too much on the one changed event, instead of all the events that led up to it.

Just as authors sometimes fail to consider the obvious "ripple effects" that might result from one major change, they often act as though a major change comes out of nowhere

The real reasons for the change might be ten years earlier, or even a hundred years.

Mixing up urban legends with actual history

Rule of thumb: If you have to look up on Wikipedia to figure out when the Hundred Years War happened, you probably shouldn't write about early modern Europe

"It ain't what you don't know that'll kill you; it's what you think you know that just ain't so." People tend to assume that they know things about the past that are in fact folk-history and quite wrong. Because they think they already know, they don't bother to look. This is a trap always waiting before our feet and nobody's totally immune to it.

Assuming that nothing will change besides your one big alteration — or that everything will

"Some brands will still exist, others won't. Some attitudes will still exist, others won't." Both sins are equally bad: Pretending that absolutely nothing will change apart from one historical event is just as unrealistic as assuming that the world will be totally different in every detail.

Making the story go where you want it to go, instead of where your altered history will support

"trying to entertain and, with luck, to provoke some thought." And when it comes down to it, you can break almost any rule in the service of a good story. Except that it does have to make sense, and Turtledove says one common sin is "making the story go where you want it to go, regardless of whether they change you've made can plausibly take you there."

Explaining too much

I suspect that the biggest problem is an author showing off their research — they did it, and therefore figure the reader will be as fascinated with the minutiae as they themselves were. Determining the pertinent details of the time-shift, and then integrating them organically is a serious challenge.

"One of the real problems with alternate history is explaining too bloody much — the 'As you know, Bob' syndrome is strong in this subgenre."

Forgetting to tell a good story

if you're not creating interesting characters and a story worth telling, then you're just "doodling in the margins of history." There has to be a point to all this stuff, beyond just the fascinating "what if" question.

Fundamentally, all stories are about the relationships between characters and when we try to paint an alternate universe with an intricate counterfactual history, too often we get swept up in the details.

The reason Pixar films are timeless and most Dreamworks animation movies will wind up in the dustbin of history is that Pixar focuses on characters while Dreamworks makes pop culture references. Like I said, this seems counter-intuitive when talking about alternate histories, but every once in a while you need to step back and remind yourself that as an author you're creating a world with characters that have motivations not unlike those that you're familiar with today.

Great alternate histories are timeless.