Stereochemistry: R and S notation

How do we number the carbon atoms in symmetrical molecules while giving R and S notation. For instance, in 2,3-Dibromobutane, there is an ambiguity over which is carbon C2 and carbon C3. Will we call it (2R,3S) or (2S,3R)?


![enter image description here](https://i.stack.imgur.com/w21zN.png)




The current version of *[Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book)](http://dx.doi.org/10.1039/9781849733069-fp001)* reads as follows:



> 
> **P-14.4** NUMBERING
> 
> 
> When several structural features appear in cyclic and acyclic compounds, low locants
>  are assigned to them in the following decreasing order of seniority:
> 
> 
> (…)
> 
> 
> (j) When there is a choice for lower locants related to the presence of stereogenic centers or stereoisomers, the lower locant is assigned to CIP stereodescriptors *Z*, *R*, *M*, and *r* (pseudoasymmetry) that are preferred to *E*, *S*, *P*, and *s*, respectively, which are preferred to the non-CIP stereodescriptors *cis*, *trans*, or *r* (reference), *c*, and *t* (…).
> 
> 
> (…)
> 
> 
> 


Therefore, the chirality center *R* receives the lowest locant and thus a systematic name is **(2​*R*,3​*S*)-2,3-dibromobutane** and not (2​​*S*,3​​*R*)‑2,3‑dibromobutane.


However, your drawing shows (2​*R*,3​*R*)-2,3-dibromobutane.


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