Le Chatelier–Braun principle analyzes the qualitative behaviour of a thermodynamic system when a particular one of its externally controlled state variables, say L , {displaystyle L, } changes by an amount Δ L , {displaystyle Delta L, } the 'driving change', causing a change δ i M , {displaystyle delta _{mathrm {i} }M, } the 'response of prime interest', in its conjugate state variable M , {displaystyle M, } all other externally controlled state variables remaining constant. The response illustrates 'moderation' in ways evident in two related thermodynamic equilibria. Obviously, one of L , {displaystyle L, } M {displaystyle M} has to be intensive, the other extensive. Also as a necessary part of the scenario, there is some particular auxiliary 'moderating' state variable X {displaystyle X}, with its conjugate state variable Y . {displaystyle Y.} For this to be of interest, the 'moderating' variable X {displaystyle X} must undergo a change Δ X ≠ 0 {displaystyle Delta Xneq 0} or δ X ≠ 0 {displaystyle delta Xneq 0} in some part of the experimental protocol; this can be either by imposition of a change Δ Y {displaystyle Delta Y}, or with the holding of Y {displaystyle Y} constant, written δ Y = 0. {displaystyle delta Y=0.} For the principle to hold with full generality, X {displaystyle X} must be extensive or intensive accordingly as M {displaystyle M} is so. Obviously, to give this scenario physical meaning, the 'driving' variable and the 'moderating' variable must be subject to separate independent experimental controls and measurements.