Colloquial stratum can be traced to several strata in what lexicon?
The earliest, colloquial stratum is traced to the Han dynasty (206 BCE - 220 CE); the second colloquial one comes from the period of the Southern and Northern Dynasties (420 - 589 CE); the third stratum of pronunciations (typically literary ones) comes from the Tang Dynasty (618–907 CE) and is based on the prestige dialect of Chang'an (modern day Xi'an), its capital.