The bellows is a device for delivering pressured air in a controlled quantity to a controlled location. At its most simple terms a bellows is a container which is deformable in such a way as to alter its volume which has an outlet or outlets where one wishes to blow air. The bellows differs from the fan in that it exerts much greater control in where it delivers the air.

Several metallurgical processes require such heat that it could only be reached after the invention of the bellows.

In musical instruments the bellows is often employed as a substitute or regulator for air pressure provided by the human lungs.

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The bellows can also refer to a length or direction adjustable tubular element; in collapsible devices or applications that require as good sealing as possible, or cannot accommodate other methods of achieving those aims.