Here ( ∂ ∂ F / ∂ ∂ T ) ( T , P , V ) denotes the (partial) derivative of the state equation F with respect to its T argument, keeping the other two arguments fixed, evaluated at the state ( T , P , V ) in question. The other partial derivatives are defined in the same way. These two equations on the four infinitesimal increments normally constrain them to a two-dimensional linear subspace space of possible infinitesimal state changes, that depends on the material and on the state. The constant-volume and constant-pressure changes are only two particular directions in this space.