A casing string consists of individual joints of steel pipe which are connected together by threaded connections. The joints of casing in a string generally have the same outer diameter and are approximately 40ft long. A bull-nose shaped device, known as a guide shoe or casing shoe, is attached to the bottom of the casing string and a casing hanger, which allows the casing to be suspended from the wellhead, is attached to the top of the casing. Various other items of equipment, associated with the cementing operation, may also be included in the casing string, or attached to the outside of the casing e.g. float collar, centralisers and scratchers. This equipment will be discussed in greater depth in the chapter associated with cementing.