There are many ways to define the proportionality constant of Henry's law, which can be subdivided into two fundamental types: One possibility is to put the aqueous phase into the numerator and the gaseous phase into the denominator ("aq/gas"). This results in the Henry's law solubility constant H s . Its value increases with increased solubility. Alternatively, numerator and denominator can be switched ("gas/aq"), which results in the Henry's law volatility constant H v . The value of H v decreases with increased solubility. IUPAC describes several variants of both fundamental types. This results from the multiplicity of quantities that can be chosen to describe the composition of the two phases. Typical choices for the aqueous phase are molar concentration ( c a ), molality ( b ), and molar mixing ratio ( x ). For the gas phase, molar concentration ( c g ) and partial pressure ( p ) are often used. It is not possible to use the gas-phase mixing ratio ( y ) because at a given gas-phase mixing ratio, the aqueous-phase concentration c a depends on the total pressure and thus the ratio y / c a is not a constant. To specify the exact variant of the Henry's law constant, two superscripts are used. They refer to the numerator and the denominator of the definition. For example, H s c p refers to the Henry solubility defined as c / p .