[Originally posted at SF Novelists.]

One of these days, I’m going to put together a little book, containing a sample of the weird metaphors writers use to describe writing.

Mostly I talk about stories in terms of textiles. To other writers I know, stories are soups or rocks or things that spin in their heads. (No, really.) But the other day, I came up with a new metaphor that I think perfectly sums up for me how flash fiction works.

It’s an arch.

The thing about an arch is, the legs of it can’t stand up on their own. The plot, the characters — flash fiction doesn’t have a lot of room to develop those. They’re pretty flimsy, by the standards of a short story (let alone a novel). In fact, a lot of the time, the core idea of a piece of flash is one that couldn’t survive at longer length; an idea that’s evocative at five hundred words may look weak or nonsensical at five thousand. So when you’re reading or writing a flash story, you’re building up these sides, and they’re curving in toward each other, and the end of the story has to come soon or the whole thing will collapse for lack of support.

But then. Then you get to that last paragraph, that last sentence, that last word — and you place the keystone. The final piece of the arch, the one that turns it from two unstable stacks into a coherent, solid structure, one that can support a great weight. The arch doesn’t work until it has a keystone; flash doesn’t work until that last bit is in place. And that, to me, is what differentiates a mediocre piece of flash fiction from a great one: if the keystone is absent or wrong for the story, then the whole thing collapses, leaving me with a sense that, eh, it was interesting while it lasted.

Get that keystone right, and the thing will stand in my mind for a long time afterward.

Any piece of fiction is improved by a good ending, of course. But the final chapter or scene of a novel is (usually) denouement, releasing the tension and returning you to the world you left behind. A short story’s tighter, but still, the moment when it comes together is often just a little bit ahead of the end, in order to play out the consequences. Flash fiction that delivers its punch a paragraph before the end? Is flash fiction that went on a paragraph too long.

Find the keystone. Build the arch, drop it into place, and transform those few hundred words into more than the sum of their parts.