As shown in the above quotation from Jean-Martin Charcot’s teaching of the late 19th century, the concept of atypical Parkinsonian disorders and formes frustes of the classic disease emerged in parallel with the definition of Parkinson’s disease itself. In 1817, James Parkinson, a London general practitioner, described resting tremor and gait impairment in the small sample of subjects whose symptoms would later be coalesced into a disorder that would bear his name (2). Nearly 50 yr later, Charcot returned to this early description and used his large patient population to study Parkinson’s disease in full detail. With access to thousands of elderly patients who lived in the sprawling hospital-city of the Hopital de la Salpetriere in central Paris, Charcot studied the evolution of signs from very early disease through the most advanced stages (3,4). Charcot used specialized recording equipment to distinguish the rest tremor of typical Parkinson’s disease from the tremors typical of multiple sclerosis and other conditions where posture- or action-induced exacerbation occurred (5). He was particularly adept in distinguishing bradykinesia as a cardinal feature of the illness and separating it from weakness. These studies led him to discourage the original designation of paralysis agitans, because patients did not develop clinically significant loss of muscle power until very late. Charcot further emphasized the distinctive elements of rigidity and delineated its distinction from spasticity or other forms of hypertonicity. Finally, he succinctly described the stance and gait of the subject with Parkinson’s disease: