The practice may sound strange, but it’s actually backed by law in several U.S. states. Instead of putting up dozens of “Keep Out” signs that can fade, fall, or get stolen, property owners use purple paint as a clear, durable warning.

It’s a legal substitute for signage in many rural and wooded areas — especially where hunters, hikers, or wanderers might cross private land without realizing it.

But there’s a right and wrong way to do it. Purple markings have to follow specific rules to count legally. The paint must be applied in vertical lines, not random smudges or horizontal streaks. These stripes need to be at least one inch wide and eight inches long, so they’re visible from a reasonable distance. The marks should sit three to five feet off the ground — low enough to catch someone’s eye, but high enough not to be hidden by tall grass or brush.