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Chest-wall compliance is the volume response of the passive chest wall to pressure across it. For a local static relation,It differs from lung compliance, which uses the pressure difference between the alveolar gas and pleural surroundings. Active respiratory-muscle effort must be distinguished from passive elastic recoil when defining the chest-wall relation.

The chest-wall relaxation volume is the passive volume at which its recoil pressure is zero. Below that volume, outward recoil corresponds to negative $P_W$ in this convention; above it, inward recoil corresponds to positive $P_W$. Negative recoil pressure is compatible with a positive slope $d{P}_W/dV$ and positive compliance. OpenStax describes chest-wall elasticity opposing the inward pull of the lungs.^[ref]

An original linear illustration makes the signs explicit. Over a chosen volume range, supposewith volume in litres and pressures in centimetres of water. Both components have compliance $0.20\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$. At $V=2.5$ L, lung recoil is $+7.5$ and chest-wall recoil is $-7.5$ cm water, so their sum is zero. At $V=3.0$ L, the sum is $10-5=5$ cm water. The same shared half-litre change requires a five-centimetre-water increment even though both component slopes are positive. These are toy relations, not measured normal curves.

Adding pressure drops givesThe combined compliance in the example is $0.10\mathrm{L}(\mathrm{cm}{\mathrm{H}}_2\mathrm{O}{)}^{-1}$. Treating that total as lung compliance alone assigns the chest-wall contribution to the wrong structure.

Pressure partition requires more information than a total volume and airway-pressure measurement. A primary lung-mechanics study's use of an esophageal balloon and differential pressure measurement illustrates the experimental effort to constrain an internal pressure reference.^[ref] Its relationship to pleural pressure, posture and muscle activity must be accounted for rather than assumed away.

A useful mechanical comparison states whether the wall is passive, which pressure zero is used and whether local or interval compliance is reported. Chest movement, total respiratory-system stiffness and the chest wall's own elastic slope answer distinct questions.