The first evidence of dialectal variation is found in texts from the Spring and Autumn period (771–476 BC). At that time, the Zhou royal domain, though no longer politically powerful, still defined standard speech. The "Fangyan" (early 1st century AD) is devoted to differences in vocabulary between regions. Commentaries from the Eastern Han period (first two centuries AD) contain much evidence of local differences in pronunciation. The "Qieyun" rhyme book (601 AD) noted wide variation in pronunciation between regions, and set out to define a standard pronunciation for reading the classics. This standard, known as Middle Chinese, is believed to be a diasystem based on the reading traditions of northern and southern capitals.