The United States Department of Agriculture uses a complicated inspection system to determine the quality rating and grade of beef. When beef is graded, a number of factors affect the evaluation. Some of these factors include firmness of the meat, color of the lean meat and its texture. The two main factors that determine the quality grade is the degree of marbling and the age of the carcass. Generally, meat that is young and has a high ratio of marbling will earn a higher rating. Marbling refers to how fat is distributed within the lean muscle. Fat is what determines how tender and flavorful a piece of beef will be. Carcass maturity also affects the quality. Younger beef will have a firm texture and a bright, red color. The grading system is comprised of eight levels, but only three, which are Prime, Choice, and Select, are widely available from supermarkets, local meat shops or restaurants. Cutter and Canner, two lower grades, are used for microwave dinners and other processed foods. Standard, Commercial and Utility graded beef are generally unlabeled, low-quality cuts of meat. Prime and Choice are considered to be of the highest quality. USDA Prime is available only at fine restaurants and certain meat markets because the price of the meat is substantially higher than other grades. Less than 3% of graded beef is marked as Prime. USDA Choice beef is still a quality meat, but it is less expensive and has a lower percentage of marbling than Prime. Because it is cheaper and has enough fat content for an appealing taste and texture, it is the most well-liked grade of beef. In conjunction with quality grades, yield grades are also used. This type of grading is used to identify the percentage of lean meat. It is determined by the total amount of external and internal fat of the carcass in relation to its weight. Yield grades range from 1 through 5 with 1 being the highest grade. A high-grade carcass will be the leanest and provide a high percentage of retail cuts.