There is general agreement among scholars that not all full vowels show similar reduction patterns. The scale is realised more clearly in A-class vowels (in open syllables) than in B-class vowels (in closed syllables); the latter generally show less reduction than A-class vowels. The scale for A-class vowels is usually assumed to correlate with vowel height and rounding: high vowels reduce less often than non-high ones, and round vowels show less reduction than non-round ones. Furthermore, diphthongs never reduce.