Dog Fence

A fence is a freestanding structure designed to restrict or prevent movement across a boundary. It is generally distinguished from a wall by the lightness of its construction: a wall is usually restricted to such barriers made from solid brick or concrete, blocking vision as well as passage (though the definitions overlap somewhat).

Fences are constructed for several purposes, including:

* Agricultural fencing, to keep livestock in or predators out
* Privacy fencing, to provide privacy
* Temporary fencing, to provide public safety and security on construction sites
* Perimeter fencing, to prevent trespassing or theft and/or to keep children and pets from wandering away.
* Decorative fencing, to enhance the appearance of a property, garden or other landscaping
* Boundary fencing, to demarcate a piece of real property

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.

Pet fences are also used sometimes to contain livestock in circumstances where ordinary agricultural fencing is inconvenient or not legal, such as on British common land.

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.