The "DOT" marking on a tire certifies that the tire meets the minimal performance standards of the U.S. Department of Transportation. The encoded letters and numbers beneath DOT are a tire's serial number. This code provides the name of the manufacturer (a brand name on the sidewall may represent only the distributor), the tire's size, the materials and date of manufacture. For example, the last three digits; 211, would mean the tire was made in the 21st week of 1991.