Core spun yarn is generally made of synthetic fiber filaments with good strength and elasticity as the core yarn, wrapped with short fibers such as cotton, wool, and viscose fibers by twisting together. The core spun yarn has the excellent properties of filament core yarn and short-spun fiber. The more common core spun yarn is polyester-cotton core spun yarn, which uses polyester filament as the core yarn and wraps cotton fiber. http://www.czsewingthread.com/core-spun-yarn/