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 side chain. 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 side chain, 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.
White Book,2nd ed., p. 43 (http://www.chem.qmul.ac.uk/iupac/bibliog/white.html)