Proto-Mandarin is a term that can be used to designate any earlier form of the most widely-spoken Chinese

Chinese language

The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...

 dialect

Dialect

The term dialect is used in two distinct ways, even by linguists. One usage refers to a variety of a language that is a characteristic of a particular group of the language's speakers. The term is applied most often to regional speech patterns, but a dialect may also be defined by other factors,...

, known in English

English language

English is a West Germanic language that arose in the Anglo-Saxon kingdoms of England and spread into what was to become south-east Scotland under the influence of the Anglian medieval kingdom of Northumbria...

 as Mandarin; it's the descendant of "late Middle Chinese" in the Song Dynasty

Song Dynasty

The Song Dynasty was a ruling dynasty in China between 960 and 1279; it succeeded the Five Dynasties and Ten Kingdoms Period, and was followed by the Yuan Dynasty. It was the first government in world history to issue banknotes or paper money, and the first Chinese government to establish a...

. "Early Mandarin" is the common name for the sound system described by the rhyme dictionary Zhōngyuán Yīnyùn (中原音韻, lit. Sounds and Rhymes of the Central Plains) of the later Yuan dynasty

Yuan Dynasty

The Yuan Dynasty , or Great Yuan Empire was a ruling dynasty founded by the Mongol leader Kublai Khan, who ruled most of present-day China, all of modern Mongolia and its surrounding areas, lasting officially from 1271 to 1368. It is considered both as a division of the Mongol Empire and as an...

, used for rhyming of Zaju

Zaju

Zaju was a form of Chinese drama or Chinese opera which provided entertainment through a synthesis of recitations of prose and poetry, dance, singing, and mime, with a certain emphasis on comedy...

(雜劇) of that time.