Sanjiu range is a concept of music theory, which refers to nine notes in western music, including C, D, treble C and so on. However, "Erjiu" is not the correct name for Sanjiu register, and it may be a misunderstanding or a slip of the tongue by some people.

Sanjiu register is one of the most basic registers in western music, which covers most musical works and playing skills. In music theory, Sanjiu register is widely used in composing and arranging music. If you hear someone call the Sanjiu range "Erjiu", it should be due to misunderstanding or a slip of the tongue.