Abstract: Stiff cords especially useful for reinforcing V-belts are made from yarns formed of extruded filaments. Almost all of the filaments have a core and a complete sheath. The proportion of sheathed filaments, A in percent to all of the sheathed filaments in the yarn, in which the proportion of the sheath in each sheathed filament is (M.+-.0.1) percent of the total volume of the sheathed filament, simultaneously satisfies the conditions A.ltoreq.100; M.gtoreq.0.5; A.gtoreq.30+(0.1 M).sup.8. The sheath is formed of a polymer having a melting point at least 10.degree. C. below the melting point of the polymer forming the core. One such sheathed filament has a core formed of polyethylene terephalate and a sheath formed of a copolymer of polyethylene terephalate and sulfoisophthalic acid.