Four hu

The four hu (Chinese: 四呼; pinyin: sì hū) are a traditional way of classifying syllable finals of Mandarin dialects, including Standard Chinese, based on different glides before the central vowel of the final. They are^[1][2]

• kāikǒu (開口, "open mouth"), finals without a medial
• qíchǐ (齊齒, "even teeth"), finals beginning with [i]
• hékǒu (合口, "closed mouth"), finals beginning with [u]
• cuōkǒu (撮口, "round mouth"), finals beginning with [y]

The terms kāikǒu and hékǒu come from the Song dynasty rime tables describing Middle Chinese.^[3] The Qing phonologist Pan Lei divided each of these categories in two based on the absence or presence of palatalization, and named the two new categories.^[4]

This traditional classification is reflected in the bopomofo notation for the finals, but less directly in the pinyin:^[a]