A meso compound is an achiral molecule despite having stereocenters. Tartaric acid, , has two tetrahedral carbon stereocenters. Its and forms are enantiomers, but and 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.[ref]
5.7 Meso Compounds. McMurry, Organic Chemistry, §5.7, Figure 5.12: meso-tartaric acid and its symmetry plane
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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A meso compound is a type of stereoisomer that has multiple stereocenters but is achiral due to an internal plane of symmetry. This means that even though it has chiral centers, the overall molecule is not optically active because one half is a mirror image of the other. Meso compounds typically have symmetrical structures, and this symmetry leads to their achirality.