Vent Reservoirs are exclusive for the wide neck 8-ounce Dr. Brown"s Natural Flow baby bottles. They are the exact pieces that come with each wide neck 8-ounce Dr. Brown"s Natural Flow baby bottle and are meant to replace worn or lost vent reservoirs. These vent reservoirs are an integral part of the Dr. Brown"s patented internal venting system. If you need to replace the entire venting system for your Dr. Brown"s Natural Flow baby bottle, you will also need Dr. Brown"s replacement Vent Inserts.