The yarn fed through at the delivery rollers only is usually known as the "core", and the other component is known as the "wrapper". The core may be of continuous-filament yarn or of spun yarn. If the core is of spun yarn, the direction of its twist is usually the same as that of the complete yarn. Core-spun yarns are made for decorative purposes or, more commonly, for strengthening the wrapper for facilitating subsequent processes. When used for strengthening, the core may, after it has served its purpose, be removed by solvent or other chemical action, e.g. the removal of calcium alginate filament yarn by an alkaline scour or of a cotton yarn by carbonising. The core is often retained for strengthening the resultant fabric as is the case if nylon or polyester continuous-filament yarns are used.