units, the majority must be fixed to the car permanently, although some units (usually smaller) are available with either magnetic or suction cup mountings which can be removed. The Light bar may also contain the Audible Warning devices. Some jurisdictions disallow the sale of lightbars to the general public (especially if mandated under state and/or federal law regardless of lens coloring which are not red and blue). Beacons - A simple lighting device, often found on smaller emergency vehicles, or unmarked cars, (where it is removable) and consists of a clear or transparent coloured casing surrounding a lamp and a revolving mirror. Some beacons consist of one or several lamps that revolve, instead of using a mirror. These were the original type of emergency vehicle lighting Alternating Vehicle Lights or Wig-wags - This causes the Full beam headlights, or Fog lights to flash in a pattern (usually alternating left-right-left, although it can be together, or in a random pattern), and can also be used at the rear of the vehicle on Brake, Fog or reversing lights to warn vehicles approaching from the rear. Grille lights - Flashing lights fitted on, into or behind the grille of the emergency vehicle, creating forward-facing flashing effects, designed to be seen in the rear view mirror of moving traffic in front of the vehicle. These can be fitted to both overt and covert emergency vehicles, by simply changing whether they are mounted internally or externally, or by varying the lens colour (so that they may look like fog lights on a covert vehicle) Dash Lights - These are forward-facing lights, like grille lights, but mounted on the dashboard of the emergency vehicle. They are more often found on covert vehicles, but may be found on some marked vehicles which are trying to increase visibility. In order to avoid dazzling the driver, they are normally fitted with 'shields' around the light which stop the light reflecting into the cab. Deck lights (or Parcel Shelf lights) - Rear-facing equivalents of the Dash Lights which are placed on the Parcel Shelf of the vehicle. These can be used in both overt and covert vehicles. Directional Warning Arrows or arrow sticks - A strip of lights (typically amber or yellow) which light up in sequence to direct traffic to the right, left, or around both sides of an emergency vehicle. They may be found mounted on the back