Fencepost Mistake

Common Mistake

Let $F$ be a fence of length $n$ units, with fenceposts at either end and spaced $1$ unit apart.

How many fenceposts are there?

To answer $n$ is to make the fencepost mistake.

There are $n + 1$ fenceposts.

Also known as

This is usually seen as fencepost error, but $\mathsf{Pr} \infty \mathsf{fWiki}$ prefers to reserve the word error for its context in unavoidable inaccuracies in measurement.

Also see

• Results about fencepost mistakes can be found here.