This is the first article of a several-part series on different aspects of a core world-building topic: consistency.

The basic message is very simple: your world must be internally consistent. Doesn’t matter if it’s science fiction or fantasy, and it makes no odds if it’s a novel, game, or opera. At all times, your world must be consistent to itself.

Now, this is very different from the behaviour of the individuals within that world being identical or them all having the same opinion (we’ll come to that in part 2), so don’t get muddled. I mean the world itself: the laws of physics and so on. These can be different from the ones in our world if you really want them to be. Changing stuff will make it hard to foresee all of the knock-on effects, but it is possible to do. For example, the science fiction novel On by Adam Roberts is based in a world where gravity runs in a different direction to the norm. Messing with this fundamental law makes a huge change to the way this world works, but that’s fine. What matters is internal consistency within the world, and he applies that with rigour. Once you get your head round this basic oddness it makes perfect sense, and the rest follows logically.

In your world, you are likely to choose to keep the laws of physics as they are in the real one where possible. This is certainly the simplest approach, and often messing with them is unnecessary. However, fantasy and science fiction genres pretty much compel you to dabble in the unreal, and this is where you need to be watchful.

Luckily, you can rely on humans. Humans are adaptable, and as most of your audience are likely to be humans you can rely on this to get away with all manner of odd situations and realities. Once they get over the initial shock of the unfamiliar, your audience will generally follow along with your fictional reality as long as it makes sense in its own terms: ie, it is internally consistent.

This gets to the why of it. Why do you need to be consistent? It’s about trust. With your fictional world you build a contract with your audience; you need them to trust you as you spin this tale. This is a layering process where you start with the basics, and what could be more basic than the laws of physics? Once they understand this then you can build on top and they will follow. If you change the ground rules every few paragraphs, then you make it very hard for them to move past this first step. Give them some solid ground to stand on, and no matter how odd it may be they will soon accept it – as long as it’s consistent. Then they can build on that to follow whatever your narrative might bring.

The only exception here is a surface inconsistency. You can present something which is apparently inconsistent, but here you want your audience to already trust you enough to know that it’s only apparently so. This technique is often the trigger for an investigation by the hero(es) or some other plot device. Why is this thing not what it should be? The implication here is that the world would naturally be consistent without someone or something (usually the narrative’s Big Bad) messing with it, and when the audience understand what is messing with it the answer will reveal that the world is indeed consistent.

So, your world must be internally consistent. If something you present to your audience is inconsistent with the world you have built then it had better be only apparently inconsistent and you have very good (and internally consistent) reasons for it hiding behind the curtain. A curtain that you will probably want to eventually pull back for your audience.