The scientific study of handedness must resolve many problems, hut the first and most fundamental is the problem of frequency. A review of the literature by Hécaen and Ajuriaguerra (1964) found incidences of left-handedness ranging from some 1 to 40%. How can laws be discovered for something so inconsistent between studies? This chapter examines the classification of handedness. The proposed solution to the problem of inconsistent frequencies is the foundation of the RS theory. The argument is that left-handedness is not a “type” whose “essence” remains elusive. It is rather a characteristic that varies continuously between strong left and strong right, with several varieties of mixed-handedness in between. The variability is the essence that has to be described and explained. It is as if height were classified as “tall” or “short”, but who was called tall or short differed for every investigator. If handedness varies continuously, like height, the problem is how to describe it in a way that will permit the construction of a reliable scale of measurement. Discrete types, right and left, are in common usage but nevertheless misleading.