The proportionality constant (h) which provides the relation between the energy (E) of a photon and the frequency (v) of its associated electromagnetic wave in the so-called Planck Relation E = hv. It is essentially used to describe the sizes of individual quanta in quantum mechanics. Its value depends on the units used for energy and frequency, but it is a very small number (with energy measured in Joules, it is of the order of 6.626 × 10-34 J·s).