Amino acids with two chiral centres were named in the past by allotting a name to the first diastereoisomer to be discovered. The second diastereoisomer, when found or synthesized, was then assigned the same name but with the prefix allo-. This method can be used only with trivial names but not with semisystematic or systematic names. It is now recommended that allo should be used only for alloisoleucine and allothreonine.