text: Two older prefixes still commonly used to distinguish diastereomers are threo and erythro. In the case of saccharides, when drawn in the Fischer projection the erythro isomer has two identical substituents on the same side and the threo isomer has them on opposite sides. When drawn as a zig-zag chain, the erythro isomer has two identical substituents on different sides of the plane (anti). The names are derived from the diastereomeric four-carbon aldoses erythrose and threose. These prefixes are not recommended for use outside of the realm of saccharides because their definitions can lead to conflicting interpretations.
  Another threo compound is threonine, one of the essential amino acids. The erythro diastereomer of it is allothreonine.