Aluminum Rectangle Tube is produced in 6063 and 6061 alloys. It has square defined corners on both the inside radius and outside radius. 6063 is the lower strength of the two alloys and is generally produced in thinner wall sizes but has better corrosion resistance and finishing qualities than 6061. 6061 has more strength for structural applications requiring high strength yet a light weight material. "Anodized" refers to a treament the material has underwent to provide a high resistance to corrosion along with a slick, smooth finish. "Mill" refers to finish as produced, no treatment.