General scenario The Le Chatelier–Braun principle analyzes the qualitative behaviour of a thermodynamic system when a particular one of its externally controlled state variables, say  changes by an amount  the driving change, causing a change  the response of prime interest, in its conjugate state variable  all other externally controlled state variables remaining constant. The response illustrates moderation in ways evident in two related thermodynamic equilibria. Obviously, one of   has to be intensive, the other extensive. Also as a necessary part of the scenario, there is some particular auxiliary moderating state variable , with its conjugate state variable  For this to be of interest, the moderating variable  must undergo a change  or  in some part of the experimental protocol; this can be either by imposition of a change , or with the holding of  constant, written  For the principle to hold with full generality,  must be extensive or intensive accordingly as  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.