Vehicle is equipped with two-piece lug nuts consisting of a physical lug nut and a decorative cap. lug nuts are 21mm in size. over time, especially in humid climates or where road salt is used, lug nuts swell between decorative cap and physical lug nut due to corrosion. swelling can cause 21mm tools including vehicle lug wrench to not fit on lug nut. this is a safety concern because if you have a flat tire in an emergency situation, you may not be able to change tires.