Table 2.

Classification of phonemes by complexity levels based on articulatory motor demands.

Complexity level a	Phoneme	Articulatory motor adjustments
Vowels
1	/ʌ, ə/	Anterior–posterior tongue movement with low elevation.
2	/a, i, u, o/	Maximally contrasted vowels based on acoustic and articulatory properties.
3	/ɜ, aI, aU, ɔɪ, ɔ /	Diphthongs require precise movement of the tongue body. The introduction of /ɜ/ gives the vowel a truncated, quadrilateral shape. /ɔ / and /a/ are distinct.
4	/I, e, ae, ʊ /	Front vowels require precise tongue–jaw configuration.
5	/ɝ/ /ɚ/	Retroflex vowels require bunching of the tongue.
Consonants
3	/p, m, h, n, w/	Quick, ballistic movements for /p,m,n/ with opening of the velopharyngeal port for /m,n/. Consistent slow movements for /w,h/.
4	/b, k, g, d, f, j/	Introduction of velars. Fast, ballistic movements for /b,k,g,d/. Consistent, slow movements for /j/. Control of the fricative /f/.
5	/t, ŋ, r, l/	Additional fast, ballistic movements for /t, ŋ/. Complex movement and bending of the tongue for /r,l/.
6	/ʈʃ, dʒ, s, z, v, ʒ, ʃ, ð, θ/	Precise movements of the tongue for dental, alveolar, and palatal placements with frication.
7	Two consonant clusters	Transition of articulatory placements requiring precision and efficiency.

^a Age at which phonemes are mastered is also considered for each complexity level. Thus, vowels start at a complexity level of 1 and consonants start at a complexity level of 3 based on the age at which the earliest vowels and consonants are mastered.