An Island Vent
An island vent rises as high as possible under an island before running to a regular vertical vent. It keeps air in the drain system and prevents siphoning of the P-trap.
Tip: If you plan to attach a garbage disposer to an island sink, replace the sanitary tee and cleanout adapter with a flat-patterned cleanout tee. That makes it easier to snake the top of the loop if it gets plugged.
Note: An island vent can be downloaded and printed from the Additional Information below.