Languages range from two to over ten vowel categories

Evidence

The number of vowel categories varies in different languages, ranging from two to a number higher than ten, depending somewhat on the classification system used (WALS, 2014). … They refer to two adjacent vowel sounds occurring within the same syllable. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 575

Categorization

• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 575