雙元音

雙元音（diphthong）是一種複元音（compound vowel），又稱雙元音或二合複元音，在語音學中，指聯合的兩個元音，它們作為一個整體出現。之間有平滑的過渡，也就是說，雙元音中的發音牽涉到兩種不同的舌位，並且從其中一種舌位滑動到另一舌位。在國際音標 (IPA) 中，單元音用一個字母表示，如[ʌ]。雙元音用兩個字母表示，如[aɪ]，響度較低的字母可以加上  ̯ 符號以表示它是雙元音的一部份，例如 [aɪ̯]。

關於拼字時兩個一組的元音字母，且通常不分別發成兩個元音的情況，請見「二合字母」。

American English pronunciation of no highway cowboys, showing five diphthongs: /oʊ aɪ eɪ aʊ ɔɪ/

雙元音的類型

雙元音依響度的變化可分為上升雙元音與下降雙元音（或稱上升複元音與下降複元音）。上升複元音以響度較低的元音開始，以響度較高的元音結束，例如 [ɪ̯a]。下降複元音以響度較高的元音開始，以響度較低的元音結束，例如 [aɪ̯]。在某些語言或研究中，會將下降複元音視為真正的雙元音，至於上升複元音則傾向於被當作相鄰出現的兩個元音。例如在漢語中，上升複元音常被當作介音與主要元音的結合，而下降複元音則被當作一個整體的押韻單位。

參見

• 音位變異（英語：Phonemic differentiation）
• 三元音

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