Reversal of asymmetry in monozygotic twins expresses itself in varying degrees, ranging from complete situs in versus viscerum in conjoined twins, to left-handedness and counter-clockwise hair-whorl in separate twins. Two kinds of handedness must be distinguished, genetic and epigenetic. Genetic right- and left-handedness seem to be dominant and recessive, respectively. Epigenetic left-handedness is a non-hereditary somatic modification resulting from twinning. There are 2 kinds of monozygotic twins: true duplicates in which there is no asymmetry reversal in 1 of a pair, and mirror-image duplicates in which there is more or less reversal in 1 of the twins. The most strikingly similar pairs show no mirror-imaging, while the less similar pairs usually show many phases of mirror-imaging. The theory offered to account for this situation is as fol-lows: The embryonic process of twinning takes place sometimes before the fixation of asymmetry in the embryo, sometimes during fixation, and sometimes after fixation. If the separation of the single embryo into twin primordia occurs before any asymmetry has been fixed, true duplicates result; if during fixation, partial asymmetry reversal occurs and various grades of mirror-image duplicates result; and if after fixation, profound asymmetry reversal occurs, involving situs inversus viscerum.