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In page Stereoisomerism:

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  • A configurational stereoisomer is a stereoisomer of a reference molecule that has the opposite configuration at a stereocenter (e.g., R- vs S- or E- vs Z-). This means that configurational isomers can be interconverted only by breaking covalent bonds to the stereocenter, for example, by inverting the configurations of some or all of the stereocenters in a compound.[citation needed]
  • An epimer is a diastereoisomer that has the opposite configuration at only one of the stereocenters.[citation needed]