Kuo-yü (in Wade-Giles) or Guoyu (in pinyin) (國語 ㄍㄨㄛ ㄩ) (literally "national language") was adopted by Republic of China (on Taiwan) and hence as the official pronunciation of Chinese language words. Technically, Kuo-yü refers only to speech and not to writing. In practice, it is synonymous with Mandarin. Unlike the People's Republic of China's putonghua, which is influenced by non-Beijing forms of Mandarin, Kuo-yü is in theory based only on the pronunciation of Beijingese (speech of Beijingers). In practice, the differences are slight.
Guoyu originally referred to the language the Emperor spoke. It was the Xianbei language during the Northern Wei Dynasty, Mongolian during the Yuan Dynasty and Manchu during the Qing Dynasty. Guoyu in the sense of national language was coined in Japan (kokugo in Japanese) and then was loaned into Chinese.
Kuo-yü is the official term on Taiwan used to refer to Mandarin. The term Kuo-yü to refer to Mandarin is rarely used in Mainland China, because declaring Mandarin to be the national language would be deemed unfair to other Chinese dialects and ethnic minorities. Instead the terms putonghua (common speech) or hanyu (language of the Han) are preferred. Some in Taiwan, especially proponents of Taiwan independence also object to the term guoyu to refer to Mandarin on the grounds the "nation" referred to in the name of the language is China and that Taiwan is or should be independent. They prefer to refer to Mandarin with the terms Beijingese or Zhongwen (writing of China). As with most things political in Taiwan, some support the name for precisely the same reasons that others oppose them.
It has small differences with Putonghua in: