A pet fence, fenceless bounary or invisible fence is a wire around the perimeter of a home, designed to keep a pet within the boundaries of the property. The wire (which may be buried) carries a radio signal, while the pet wears a lightweight receiver collar which emits a warning sound when the pet nears the boundary. If the warning is ignored and the pet crosses beyond the boundary of the fence, the pet receives a mild electric shock. The pet soon learns to avoid the invisble fence location, making it an effective barrier. Animals not wearing the collar (including humans) are unaffected by the underground wire. Although called "fences", these fenceless boundary systems are more accurately termed electronic pet containment systems.