A pet fence, fenceless boundary 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 invisible 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.
Some pet fences are wireless. Rather than using an underground wire, they emit a radio signal from a central unit, and activate when the pet travels outside a set radius from the unit.
In another type, the collar uses GPS signals to determine proximity to a predetermined "virtual fence", without the need for any physical installation at all. This system allows some additional flexibility, such as simpler inclusion of "islands" within the containment area, and easier changes to the boundary.