From studies on the movement of position, in space, of a fiber, of which yarn is constructed, in the process of single and plied yarn twisting, a theory of twist structure has been developed. On the basis of this theory, theories of twist shrinkage, breaking strength, twist moment and stiffness in twisting of twisted yarns are developed. Experiments have been made with rayon tire yarns and rayon tire cords, and the results, theoretical and experimental, agree fairly well.