The pronunciation of the borrowed Chinese characters in Japanese, Korean, and Vietnamese also provide valuable insights. Old Chinese was not wholly uninflected. It possessed a rich sound system in which aspiration or rough breathing differentiated the consonants, but probably was still without tones.
The study of how borrowed Chinese characters are pronounced in Japanese, Korean, and Vietnamese can provide important information. Ancient Chinese lacked full inflection but had a sophisticated sound system that included distinctions through aspiration or rough breathing, though it did not have tones.