The sill is a panel or edging which forms part of the car body, running beneath the side doors of the car.

The sills can be welded into place beneath the side doors on both sides of the car, or fixed with a strong sealer or other adhesive, or the use of rivets and screws. Sills are usually manufactured in stainless steel, aluminum, carbon fiber plastics, fibreglass or polyurethane polymers, but the materials used, as with the particular design, colour, finish and fittings will be dependent on the specific car make and model. Sometimes sills display the car manufacturer's logo or the make and/or model of the vehicle.

Many car owners choose to fit new sills on their vehicle for aesthetic reasons; they can enhance the design of the car, giving it a lowered look, making the car appear closer to the road, which gives a sporty edge and stylish finish. As well as offering a customised look, sills also function to block and redirect the air flow, as the car is travelling, reducing the amount of air going underneath the car, which reduces drag and underbody lift and enables the car to travel faster while improving fuel efficiency and overall handling and stability of the vehicle. 

Poor fitting parts, such as the sills on a vehicle, will affect the performance of the part and leave unsightly gaps and rough edges and you will lose that contoured continuity that you get with a part that is made to fit the make and model of vehicle. sill will ensure that the sill you install will be completely compatible with the make and model of your car, providing a sleek and contoured fit, instantly adding style and functionality.  Although constructed from materials which are strong, rigid and absorb impact well, the sills on a vehicle will eventually need replacing either due to general wear and tear, rusting, warping or cracking possibly over time, or because of damage suffered in a collision.