Residential chimneys with walls thinner than 8 inches (nominal) must be lined. Since the early 1900s, clay tile has been used for this purpose, but in recent years, due to the cost, masonry has faced stiff competition from other systems. However, clay flue liners, which can last 150 years, are far more durable and still are commonly used in higher-end home construction. Flue liners made from fire clay, shale, surface clay or a combination of these materials contain combustion byproducts and prevent them from penetrating the surrounding masonry. Dense, nonporous clay liners are impervious to chemical attack, corrosion and material breakdown. These liners are available in various lengths and four standard shapes: rectangular nonmodular, rectangular modular, round, and oval. Descriptions of clay flue liners from four different manufacturers are included with the article. Fired at 2000 degrees F, liners resist breakage. Prefabicated offsets with smooth ends make flue installation easier. A complete line of liners, 3- to 36-inches round with socket or self-aligning joints, square and rectangular flues, contoured rings, flues with holes and flue tees can be found. Clay flues come in a wide selection of sizes as well.