 Note that modern linguistic descriptions of Middle Chinese often refer to the level, rising and departing tones as tones 1, 2 and 3, respectively.By the time of the Mongol invasion (the Yuan dynasty, 1279–1368), former final stops had been reduced to a glottal stop in Mandarin.The "Zhongyuan Yinyun", a rime book of 1324, already shows signs of the disappearance of the glottal stop and the emergence of the modern Mandarin tone system in its place.