In the most commonly cited simple example, an ideal gas, the thermodynamic variables would be any three variables out of the following four: amount of substance, pressure, temperature, and volume. Thus, the thermodynamic state would range over a three-dimensional state space. The remaining variable, as well as other quantities such as the internal energy and the entropy, would be expressed as state functions of these three variables. The state functions satisfy certain universal constraints, expressed in the laws of thermodynamics, and they depend on the peculiarities of the materials that compose the concrete system.