Have you ever been walking in a quiet neighborhood or on a trail in the country and noticed a purple fence? Not a regular brown or white fence, but a purple fence like someone dipped it in grape Kool-Aid? Your brain says, okay, why? Is it just an odd choice of color? An odd taste in paint? As it turns out, no. There is a reason. Purple on a fence means one thing: no trespassing.

I know, a little odd. Like, why purple? Why not red, or neon orange, or a literal “keep out” sign? But in many rural areas, purple paint has emerged as a coded signal. In some states, it is even legally recognized. Rather than putting up 25 signs that could blow away or fade, you put a purple mark on a fence post and boom, the message is communicated.

Now, just because you slapped a little purple on a board, don’t expect everyone to understand. There are rules regarding the paint, depending on where you live. The major one is that the purple marks need to be vertical. Not random dabs of paint, not horizontal graffiti, but vertical stripes. This is how someone knows it is intentional and not just a bored teenager with leftover paint.

Also, size matters. Each stripe should be at least one inch wide and roughly eight inches long—large enough to catch your attention from a fair distance. What’s the point if it’s so small you don’t notice it, or it just looks like a mistake?