This is a term that is a holdover from the early days of the oil business. It refers mainly to casing and tubing but also can refer to line pipe and other pipe used in producing or transporting gas and oil. Casing is the pipe that is used while drilling the well. It is placed in the well, cemented in place, and is what keeps the hole from sloughing in while drilling or producing. It “cases” the open hole. It comes in sizes ranging from 30” to 3 ½” in diameter. Usually several “strings” of casing are using in each well. Tubing is the pipe that is inserted in the well during well completion operations. This is the pipe by which the oil and gas flows to the surface. It is frequently removed from a well during workover or completion operations. Tubing comes in sizes from 4 ½” to 1 ½” diameter. Casing and tubing each use special connectors in order to screw the joints together. These connectors are called connections and come in all types of sizes and thread profiles depending on the intended use. A joint of tubing or casing is made up of the “tube” and the “connection.” Connections are sometimes separate items called couplings or they can be integral or a part of the tube. Again, the intended use dictates that type of connection. Line pipe is a separate type of tube from tubing and casing. Line pipe is what makes up a pipeline. Pieces are connected together by welding. Line pipe is less expensive and less rigorously used than tubing or casing. Companies that manufacture these items are US Steel, Tenaris, Vallourec, Sumatomo Steel, and others.