Wu Yin and Si Hu, or Five Yin and Four Hu, are terms for Chinese opera. They are about the proper pronouncing skills.

Yin means note in pronunciation. Five Yin, the five notes of the ancient pentatonic scale, refers to the five places where airstreams pass through to make different sound. It includes Hou (throat) Yin, Chi (through teeth) Yin, Ya (teeth) Yin, She (tongue) Yin and Chun (lip) Yin.

Four Hu, referring to the proper shapes of the performer's mouth when pronouncing, falls in four categories -- Kai Hu, Qichi Hu, Hekou Hu and Cuokou Hu.

Only when the performer gets full understanding and control of Wu Yin and Si Hu, can he make the proper and articulate pronunciation when performing.

Nowadays, when we say someone is Wu Yin Bu Quan (an idiom), we mean that this person is not good at singing at all.