The present study is a description of Mandarin Chinese of the present time, approximately the language of the middle of the twentieth century. Frequent reference will, however, be made to earlier stages of the language, often reflected in older forms or older distinctions in other dialects, whenever it is relevant to the description of the present. The idea of a purely synchronic stage of a language is methodologically comparable to the idea of velocity or of acceleration at a given instant. The instantaneous value is defined as the limit of average values over a length of time, including the instant, as the length vanishes. In a sense, then, it is a scientific fiction set up for methodological convenience.
A distinction is often made between synchronic and historical studies of a language. Logically, one can study the language of one period, say that of the language of Ch’angan around 600 A.D., which will be both synchronic and historical. There is also lack of logic in the opposition sometimes set up between synchronic and descriptive studies, since diachronic studies of a language can be pursued through various stages from a descriptive point of view.
Chao Y R. A grammar of spoken Chinese[M]. Univ of California Press, 1965.