The nitrogen atoms of the imidazole ring of histidine are denoted by pros ('near', abbreviated π) and tele ('far', abbreviated τ) to show their position relative to the @B00720@. This recommendation arose from the fact that two different systems of numbering the atoms in the imidazole ring of histidine had both been used for a considerable time (biochemists generally numbering as 1 the nitrogen atom adjacent to the @B00720@, and organic chemists designating it as 3). The carbon atom between the two ring nitrogen atoms is numbered 2 (as in imidazole), and the carbon atom next to the τ nitrogen is numbered 5. The carbon atoms of the aliphatic chain are designated α and β. This numbering should also be used for the decarboxylation product histamine and for substituted histidine.