The essential source book for ancient Chinese medical theory is the Inner Classic of the Yellow Emperor – the Nei Ching. This is a collection of texts by unknown authors compiled between 300 and 100 BC. Later writers commented on the Nei Ching to bring out and clarify its ideas and sometimes to add new ones. Without such commentaries the Nei Ching would be almost incomprehensible to modern readers. Modern textbooks are based on works written in the Ching (Manchu) period (1644-1911) and before that on the Han period (202 BC to 220 AD). Much of traditional acupuncture as understood today, therefore, is not of vast antiquity.