Evidence for dialectal variation is found in texts from the Spring and Autumn period (722–479 BC). At that time, the Zhou still defined a standard speech. The "Fangyan" (1st century CE) studies the differences in vocabulary between regions. Texts from the Eastern Han period also discuss local differences in pronunciation. The "Qieyun" rhyme book (601) noted wide variation in pronunciation between regions. It wanted to define a standard pronunciation for reading the classics. This standard is called Middle Chinese.