Balance Stage II vapor recovery nozzles must make a vapor-tight connection with the fill pipe of the vehicle tank that is being fueled.
The bellows device is used to provide this tight connection is an accordion-like sleeve, made of a flexible rubber or plastic compound, that fits around the outside of the nozzle spout. When the spout is inserted into the fill pipe, the folds on the bellows compress, and the end of the bellows mechanism forms a snug, vapor-tight union with the lip of the fill pipe.
The bellows is also referred to as the nozzle boot or boot assembly.
See also Nozzle.