Two systems are said to be in the relation of thermal equilibrium if they are linked by a wall permeable only to heat, and do not change over time.[1] As a convenience of language, systems are sometimes also said to be in a relation of thermal equilibrium if they are not linked so as to be able to transfer heat to each other, but would not do so if they were connected by a wall permeable only to heat. Thermal equilibrium between two systems is a transitive relation.