Competitive antagonists are used by pharmacologists in a definitive manner in the identification of receptor subtypes. However, despite the many decades of their use, there is still some confusion over terms and terminology. Don Jenkinson, a member of the IUPHAR Committee on Receptor Nomenclature, points to terms that are often used imprecisely: pA2 is frequently used interchangeably, but erroneously, for log KB; the term 'competitive' itself has been used ambiguously, and it is even possible that confusion can arise by the expression of the constant from the Schild equation in different ways (affinity/equilibrium/dissociation constants).