The letters "DW" are visible on the second tread rib from the outer shoulder. When both letters are visible it means that the tire has enough tread to perform well in dry and wet conditions. However, when the letter "W" disappears and only the "D" is visible, the tire is no longer safe to use in wet weather, only on dry surfaces. This makes sure the driver knows when the tire loses its great wet weather traction. Once the "W" disappears, the tire is dangerous to use in wet weather.