The central puzzle addressed by the RS theory is the relationship between two lateral asymmetries, hand preference and speech hemisphere. The puzzle was outlined in chapter 1. It was shown that all of the logically possible answers to the question of the speech hemisphere of left-handers have been offered, right, left, either or both. Studies of patients with known speech hemisphere, diagnosed by the Wada test, revealed that left-handers may indeed have right or left or bilateral speech, but that right-handers may have all these speech lateralities also. If relationships between hand and brain are to be understood as a feature of human biology, their distribution must be studied in patients that are representative of the general population. However, when the distribution of speech laterality in right- and left-handers was examined in five large representative series the findings were very variable (Zangwill, 1967). My approach to this puzzle was to examine the five series in some detail so as to locate the source of the variability. As might be expected, in the light of the analysis in chapter 2 above, most of the variability was due to different classifications of left-handedness. Underlying these superficial differences were some important constant features. The constant values can be used to estimate parameters of the RS model. This chapter explains the RS solution to the puzzle of handedness and CD. It builds on the analyses of the previous two chapters, and provides the foundation for the genetic analyses to follow.