This paper shows that 16th century Pekingese already had the tone sandhi rule which converts two third tones into a second tone followed by a third, and that it operated then as a dissimilating process with respect to contour. The Neutral Tone Rule of Modern Pekingese can also be traced back to the 16th century in essentially the same form. 16th century tone values are reconstructed, and Modern values are derived from them via a hypothetical pull chain, in which only one tone feature changes at a time.