The earliest Chinese written records are Shang dynasty-era oracle bone inscriptions, which can be dated to 1250 BCE. The phonetic categories of Old Chinese can be reconstructed from the rhymes of ancient poetry. During the Northern and Southern dynasties period, Middle Chinese went through several sound changes and split into several varieties following prolonged geographic and political separation. "Qieyun", a rime dictionary, recorded a compromise between the pronunciations of different regions. The royal courts of the Ming and early Qing dynasties operated using a koiné language (Guanhua) based on Nanjing dialect of Lower Yangtze Mandarin.