A cable includes a cable sheath in which are embedded a plurality of insulated conductors at least two of which are helically twisted, the cable being provided with visible markings that indicate where one can electrically engage a selected one of the conductors by an insulation-piercing contact. The cable is formed by arranging a plurality of insulated conductors in a light-transmitting synthetic plastic sheath layer, two or more of the insulated conductors being helically twisted about a longitudinal axis, which cable is optically scanned to determine locations at which contiguous portions of the twisted insulated conductors are superposed orthogonally relative to a reference plane, whereupon markings are formed on the cable sheath layer at locations laterally spaced from the contiguous conductor portions, whereby a selected one of the twisted conductors may be engaged by an insulation-piercing contact.