internal plane of symmetry

Meaning: An internal plane of symmetry (a mirror plane) divides one molecule into two halves that are reflections of each other. The molecule is superimposable on its mirror image, so it is achiral and does not rotate plane-polarized light. With two stereocenters, the plane makes them mirror images, one R and one S, so their optical effects cancel. The result is a meso compound, such as meso-tartaric acid (2R,3S) or cis-1,2-dimethylcyclopentane.

On the MCAT: You are given a structure with two or more stereocenters and asked whether it is chiral, meso, or optically active. Check whether the stereocenters carry the same substituents with opposite configurations; if so, rotate about the central bond to an eclipsed conformation and look for the plane. Meso forms lower the stereoisomer count below the 2ⁿ maximum. A meso compound is a diastereomer of the chiral R,R and S,S forms.

Common confusion: Having two or more stereocenters does not guarantee chirality (a molecule with exactly one stereocenter is chiral). The mirror plane decides it. An R,S label alone does not prove a compound is meso: (2R,3S)-2-bromo-3-chlorobutane has no internal plane because its centers carry different substituents, so it is chiral.