Threo compounds aldol diastereomers

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

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