According to traditional phonetics, vowel ong is classified as closed call and vowel ONG as clamped call.

The definition of four tones is as follows: (1) The main vowels are A, O, E,? ,? ,? No rhythm; (2) Homophones are vowels with I as the main vowel and ending in I; (3) The closed call is a vowel with U as the main reason and ending in U; (D) Cuochou is a vowel with y as the main reason and y as the end. The rhyme of ong is actually u, and the rhyme of ONG is y, which can be seen clearly if you use the international phonetic alphabet. The reason why it is written as ONG ONG is for the neatness of Chinese phonetic system.