= R and S configuration
{c}
{title2=$R,\ S$}
For a tetrahedral <stereocenter>, rank its four <substituents> using <stereochemical sequence rules>. With lowest-priority group pointing away, a clockwise $1\to2\to3$ sequence is $R$ and a counterclockwise sequence is $S$. Looking from the other side reverses the apparent sequence, so the viewing condition is essential. The geometric letters A/B/C/D in the <tetrahedral mirror-image test> carry no assigned priority. Neither $R$ nor $S$ determines positive or negative <optical rotation>.<Organic Chemistry by John McMurry/5.5 Sequence Rules for Specifying Configuration>[McMurry, Organic Chemistry, §5.5, Figure 5.8 and discussion after Figure 5.10]{ref}
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