Historically, two conventions for sign for the electrode potential have formed:
# convention "Nernst–Lewis–Latimer" (sometimes referred to as "American"),
# convention "Gibbs–Ostwald–Stockholm" (sometimes referred to as "European").
In 1953 in Stockholm IUPAC recognized that either of the conventions is permissible; however, it unanimously recommended that only the magnitude expressed according to the convention (2) be called "the electrode potential".  To avoid possible ambiguities, the electrode potential thus defined can also be referred to as Gibbs–Stockholm electrode potential. In both conventions, the standard hydrogen electrode is defined to have a potential of 0 V. Both conventions also agree on the sign of  for a half-cell reaction when it is written as a reduction.
The main difference between the two conventions is that upon reversing the direction of a half-cell reaction as written, according to the convention (1) the sign of  also switches, whereas in the convention (2) it does not. The logic behind switching the sign of  is to maintain the correct sign relationship with the Gibbs free energy change, given by  where  is the number of electrons involved and  is the Faraday constant. It is assumed that the half-reaction is balanced by the appropriate SHE half-reaction. Since  switches sign when a reaction is written in reverse, so too, proponents of the convention (1) argue, should the sign of . Proponents of the convention (2) argue that all reported electrode potentials should be consistent with the electrostatic sign of the relative potential difference.