How many stereoisomers does cyclohexane-1,2,3-triol have? Treat the ring as planar and count cis/trans isomers, with each distinct compound or enantiomer counted once.

Three ring carbons carry OH, giving 8 up/down combinations. All-cis is meso, and the isomer with C2 trans to both C1 and C3 (C1 and C3 cis) is also meso, with C2 acting as a pseudoasymmetric centre. The isomers with C1 or C3 alone trans to the others form one enantiomeric pair, and orbit counting under the $180^{\circ }$ in-plane flip gives 4 in total.