erythro, threo

Descriptors of the diastereoisomers of an acyclic structure or partial structure having two stereogenic centres. This notation is derived from carbohydrate nomenclature. The extension of this system has given rise to conflicting interpretations of these prefixes. It is recommended that for such cases the $l$ , $u$ or $R *$ , $S *$ system should be used.

Source:

PAC, 1996, 68, 2193 (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2208