An anachronism is bad if it pulls the reader out of the story. Unfortunately, this rule isn’t much help to an author, since it’s so heavily dependent on the subjective impressions of the reader. And some readers and viewers are just picky. I would submit that if you can’t enjoy Gladiator because Russell Crowe has stirrups or if you can’t see Zulu without moaning about Michael Caine’s Webley revolver, then that probably says a lot more about you than it does about those movies.
As authors of traditional high/epic fantasy novels, we spend a lot of time reading up on the Middle Ages and Renaissance. Our hope is that we will be able to create a world that feels authentically old, but of course we know we’re going to get some things wrong. Early on, therefore, we started talking about what we could and couldn’t get away with.
First we tried to represent a specific time and place: the 15th century in England. That way, if we wanted to know what type of food our characters would eat, we’d just have to go look up what food people in England were eating in the 15th century.
The trouble is that even very good, highly detailed reference works are not quite as specific as that, particularly if you’re looking for information on non-military topics. You’ll be reading along about cooking or textiles in the 1400s, and then the next paragraph will start, “In 1528…,” or there will be a digression into something that happened during the First Crusade (1096-99).
So we had to use a much broader swath of history. But once we let ourselves be vague about the time period, how could we decide what we could use and what we couldn’t? Could we just throw up our hands and let the Myrcians have zippers, credit cards, and breakfast cereal?
We decided that, in general, technological anachronisms were worse (or at least more noticeable to the lay reader) than conceptual, philosophical, or artistic anachronisms. In other words, there are 1) things that didn’t exist at a certain time period because the society literally lacked the technology to produce them, and 2) things that could easily have existed, if only someone would have thought of them. We decided to allow anachronisms in the second category, but not the first.
According to this rule, Myrcia can have ballet, but not electric guitars. In our world, ballet started in Renaissance Italy, though the kind of dancing we associate with the term today is mostly a product of the 18th and 19th centuries. There’s nothing that would have stopped medieval English dancers from trying to dance that way, however. They just didn’t happen to think of it. Obviously an electric guitar is different, since you have to know what electricity is before you can even imagine such an instrument.
Still, anachronisms must be paid for. You have to make sure to follow through with the implications. If a country has ballet or opera, then it also probably has a more highly developed system of professional music and theater than you would have found in 15th century England. If you have ballet, then you have ballet schools. You have ballet troupes and big theaters for them to perform in. You have wealthy patrons who can front you the money for all this, and you have relatively sophisticated audiences who are willing to buy tickets to watch people prance around stage in tights for several hours.
Anyway, that’s our rule. There are some gray areas, of course. And I’m sure there will be readers who sniff and scowl and complain that we clearly don’t know how medieval kitchens worked, or whatever. But our system works for us, and so far I think it’s resulted in a pretty coherent secondary world.