There is an extensive range of different spinning systems, not all of which are in wide commercial use; many are still experimental or, having reached the commercial stage, have been withdrawn from the market. A classification of the better known spinning systems is given Table 6.1, in which the various techniques are grouped according to five basic methods. In the first section of this chapter, we will consider the fundamental principles of these listed spinning systems. In the sections that follow, we will deal with the yarn structure and properties of only those that still have commercial significance. Often, two or more yarns are twisted together to improve yarn properties or to overcome subsequent processing difficulties in, for example, weaving and knitting. The operating principles of the more common plying systems will also be described in this section.