The descriptors erythro and threo are used to distinguish between diastereomers containing only two chiral centers, which are adjacent and bearing two pairs of common ligands (1).

If, in the Fischer projection, like ligands are on the same side of the bond linking the chiral centers, the compound is identified as the erythro isomer; if they are on the opposite sides, the compound is identified as the threo isomer.

eg:  2,3-dichlorobutane



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