Although a structure with two stereocenters may have up to four possible stereoisomers, if one of the diastereomers is of this type of compound, there will only be a total of three stereoisomers.  
Examples of this type of compound include 2,3-dibromobutane and 1,3-dimethylcyclohexane.   Although their trans isomers exist as pairs of enantiomers, their cis isomers have a plane of symmetry.  Although a structure with two stereocenters may have up to four possible stereoisomers, if one of the diastereomers is of this type of compound, there will only be a total of three stereoisomers.  FTP, identify this type of compound that, despite containing tetrahedral atoms with four different attached groups, still has a plane of symmetry and is therefore achiral and optically inactive.
Examples of this type of compound include 2,3-dibromobutane and 1,3-dimethylcyclohexane.   
meso compound