With a perimeter wire, you can block a large part of your lawn. This way, your robot lawn mower knows which parts of the lawn it should mow. It's possible that the included wire isn't long enough for the surface of your garden. In that case, you can expand the perimeter with an extra perimeter wire. An extra wire is also handy if you have a large pond that requires extra wiring, for example. We have different perimeter wires with different lengths in our assortment.