Mandarin' refers to Standard Chinese or Standard Mandarin language, which is based on the particular Mandarin dialect spoken in Beijing. In its broader sense, Mandarin is a diverse group of dialects spoken in northern and southwestern China. The prevalence of this linguistic homogeneity in northern China is largely the result of geography: much of northern China is covered by plains and is flat. In contrast to this, the mountains and rivers of southern China have promoted linguistic diversity. Chronologically, there is no clear line to mark where Middle Chinese ends and Mandarin begins; however, the 'Zhongyuan Yinyun', a rhyme book from the Yuan Dynasty, is widely regarded as a milestone in the history of Mandarin. In this rhyme book we see many characteristic features of Mandarin, such as the reduction and disappearance of final stop consonants and the reorganization of the Middle Chinese tones.