= Meso compound
{wiki}
A <meso compound> is an <achiral> <molecule> despite having <stereocenters>. <Tartaric acid>, $\mathrm{HOOC{-}CH(OH){-}CH(OH){-}COOH}$, has two tetrahedral <carbon> <stereocenters>. Its $(2R,3R)$ and $(2S,3S)$ forms are <enantiomers>, but $(2R,3S)$ and $(2S,3R)$ name the same meso form after equivalent ends are identified. A symmetric spatial arrangement has a mirror plane exchanging its two halves. Thus there are three <stereoisomers>, not four.<Organic Chemistry by John McMurry/5.7 Meso Compounds>[McMurry, Organic Chemistry, §5.7, Figure 5.12: meso-tartaric acid and its symmetry plane]{ref}
Opposite <R and S configurations> alone do not guarantee a <meso compound>: the rest of the <molecule> must permit mirror superposition. Likewise, a <meso compound> is one <achiral> <chemical compound>, whereas a <racemate> is a mixture of two <chiral molecule> populations. Both can give zero <optical rotation> for different reasons.
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