The @R05260@ at two contiguous carbon atoms in the main chain bearing, respectively, substituents a and b (a ≠ b), is designated by the prefix erythro or @E02212@, as appropriate, by analogy with the terminology for carbohydrate systems in which the substituents are OH. Examples: Similar systems in which a higher level of substitution exists may be treated analogously if the erythro or @E02212@ designation is employed to denote the relative placements of those two substituents, one for each @B00574@ carbon atom, which rank highest according to the @ST06775@ Rule.