This reducing adapter lets you use a smaller bowl on your bong. For instance using the 18.8 adapter you can use 14.5mm bowls on your 18.8 bong. Using the 29.2 adapter you can use your 18.8 bowls on your Zumo Bong. If you bought both you could use a 14.5mm bowl on a roor Zumo Bong. You get the idea! These roor reducing adapters are a great little addition to any glass collection as they let you use a range of bowls in a range of roor bongs and other glass smoking pipes that use the glass joints.