The On-Stage SSA21 Subwoofer Adapter's female M20 threading attaches to M20 speaker shafts to accommodate virtually any subwoofer on the market, utilizing both removable M20 threading and a tapered, steel-lugged lower section to accept 13/8" cups. Whether it is a shallow M20, recessed M20, 1 3/8" cup or 1 3/8" cup with M20, the SSA21 will fit your subwoofer.