ONLY for 1 and 2 oz bottles - not 4 oz.. For use with bottom feed airbrushes, swap out the standard airbrush makeup bottle caps with these and be ready to go upon arrival at your gig. Fits perfect on 1 and 2 oz bottles, 20ml in size. This model has a larger airhole than many to allow more air into the bottle for use with low air pressure and less clogging. Many artists leave these caps on between gigs and simply cover the air hole with tape to prevent evaporation. Sometimes if the fitting is loose, put masking tape around the fitting for a snug fit. Cut the tube at an angle so that it does not touch the bottom of the bottle.