The diaphragm can be considered as 2 muscles, 1 which displaces both rib cage and abdomen in the inspiratory direction, and another which displaces only the abdomen. A theoretical analysis indicates that although the 2 parts are arranged as muscles mechanically in parallel, or as pumps pneumatically in series - and therefore the force and pressure developed by the diaphragm is the sum of the forces and pressures developed by each part - no anatomical site can be specified where a pressure can be measured to partition transdiaphragmatic pressure (Pdi) into its 2 components. However, a pneumatic analogue of the inspiratory muscles and the structures they displace allows for the construction of a pressure-balance equation between the rib cage and abdomen during a pure diaphragmatic Meuller maneuvre. This allows an expression which partitions Pdi in terms of the ratio of pleural to abdominal pressure changes, the ratio of abdominal to rib cage compliance, and the area of apposition of the costal fibres of the diaphragm to the rib cage. The first 2 are constants during pure diaphragmatic Meuller maneuvres. Published data for the 3 quantities suggest that the component of Pdi which acts to inflate the rib cage is only a small component of total Pdi.