Duncan made a number of Whistler yo-yos from the late 1930's and 1950's. Made out of tin, each halve had an inner and outer shell, with the halves joined by a metal axle. Whistling holes, 0.25" in diameter, were placed on the outside of the halves, allowing air to rush through the holes as the yo-yo is spinning. This created a whistling and/or humming sound. Due to the uneven number of holes on each halve (2 and 4), two different sound frequencies were created by each of the halves, allowing switching of tones as the spin slowed down. The yo-yo featured lithographed decals, typically adorned by a pattern.