These food grade silicone stoppers feature several vent holes lightly sealed with a silicone top flap. Gases can escape, but cannot get in. Just put the stopper on your carboy, and forget it. No worries about evaporation or water getting into your fermenter. This #10 size is pictured at right above, and fits all Fermonster lids, and other containers with a 1.75" opening. This is 1 5/8" diameter at the bottom, and 1 7/8" diameter at the top (right below the lip).