The relative configuration at two contiguous carbon atoms in the main chain bearing, respectively, substituents a and b (a ≠ b), is designated by the prefix erythro or threo, 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 threo designation is employed to denote the relative placements of those two substituents, one for each backbone carbon atom, which rank highest according to the Sequence Rule.