The relation is investigated between the wave number k(0) of a twisted polypropylene filament and the wave number k(1) of the double helix composed of the filaments. The sum of the wave numbers is zero, illustrating the rule of wave number conservation unless the product t = ka of the wave number k and radius a of the filament is large. The deviation from the rule At is proportional to t(0)(3) = k(0)(3)a(3), and the proportionality constant gamma is equal to the elasticity/rigidity ratio E/G.