The standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. A degree sign (°) or a superscript Plimsoll symbol () is used to designate a thermodynamic quantity in the standard state, such as change in enthalpy (Δ H °), change in entropy (Δ S °), or change in Gibbs free energy (Δ G °). The degree symbol has become widespread, although the Plimsoll is recommended in standards, see discussion about typesetting below.