The document discusses different theories of fiber structure. It defines three levels of fiber structure organization - organo chemical, macromolecular, and supermolecular. The supermolecular structure determines unique fiber features like crystallinity and orientation. Theories discussed include the micellar theory, continuous theory, fringed micelles theory, and fringed fibrils theory. The fringed fibrils theory proposes that crystalline fibrils run along the fiber length, connected by polymer chains traversing between fibrils and forming amorphous regions.