A reinforcement system for pneumatic tires featuring combination cords of individually varying extensibility composed of elements of extensible character and, combined therewith, elements of relatively inextensible character; the combination cord of greatest extensibility being preselectively located in the region of the tire subjected to the greatest deflection whereby the stress incurred during average environmental usage or of a particular impact is not transferred to the elements of relatively inextensible character until the deflection has reached a value preselectively determined. The combination cords include an organic and a somewhat greater length of substantially inextensible material, e.g., glass, so combined therewith as to yield a unit length, and are tailor-made to exhibit a particular degree of elongation depending upon the particular combination of variable lengths and the selection of organic elements to be combined with the glass.