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I was reading the Wikipedia page for vietnamese phonology and in the vowel chart, it says "/iə̯/ ⟨ia~iê⟩ /ɨə̯/ ⟨ưa~ươ⟩ /uə̯/ ⟨ua~uô⟩. What do the tildes mean?
A tilde generally indicates alternation or variation: "all of these options are possible and acceptable". In angle brackets, it indicates variation in spelling. So, ⟨ưa⟩ and ⟨ươ⟩ are both possible spellings of /ɨə̯/.
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