what is a billet grille?

In automotive engineering, a grille is an opening in the bodywork of a vehicle to allow air to enter. Most vehicles feature a grille at the front of the vehicle to allow air to flow over the radiator and cool the engine compartment. Other common grille locations include below the front bumper, in front of the wheels (to cool the brakes), in the cowl for cabin ventilation, or on the rear deck lid (in rear engine vehicles).
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Billet grilles are an aftermarket part that is used to enhance the style or function of the original OEM grille. They are generally made from billet, solid bar stock aircraft grade aluminum or stainless steel, although some are CNC Machined from one solid sheet of aluminum.
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Source: Wikipedia (where else?? hehe)