For use with any O-ring type harness as well as all the great ones from Sportsheets, the Rubber O-Ring pack lets you attach toys of different sizes to your strap-on system. Included in the 4 pack you'll find a 1.25, 1.5, 1.75 and 2 inch rings made from hardy, durable rubber. Pick the one that holds your toy of choice snug, and strap it on for a great fit.