A self-consistent geometry of cords made from multi-filament yarns is derived from the assumption that all filaments in a cord follow an epihelical path. This path is defined mathematically using a definition of ply twist not considered before, which leads to simpler formulae than those developed by previous workers. Formulae for changes in ply length, ply twist and cord length on cording are derived which enable the retraction of cords and yarns on twisting to be predicted accurately. An experimental method of cord twisting (simple twisting) is described which enables a simple experimental verification of this theory of twist to be carried out.