The teacher who uses Pinyin without knowing about such things as the place of the tone symbol in the case of polyphthong finals is merely incompetent but the teacher who supports the student in developing false notions about the language norm by adopting a systematically casual attitude to its technical aspects, can cause much greater harm.
Source: wiktionary
A syllable is short when ending in a short monophthong or in a short polyphthong; a syllable is long when ending in a consonant or in a long monophthong or a long polyphthong.
Source: wiktionary
With small preliminary he told Russ and me something like the following - I have normalised his speech in the interests of general sanity, though his polyphthongs ('hayyut' for 'hat', 'bawun' for 'born', etc.) were of the rarest quality.
Source: wiktionary