"Alliteration, cognate alliteration (p-b, f-v, ch-j, etc.), and assonance are present in almost all of
[Dylan Thomas's] poems, as well as examples of phonemic augmentation, diminution, and chiasmus. [Moynihan's footnote states:] [Kenneth] Burke defines "cognate" letters as letters which
are related to one another by the position of the mouth in pronouncing them. Thus m, b, and p are cognates of one another but not of z or k. 'Augmentation' and 'diminution' are terms which Burke has borrowed from music where a theme in quarter notes may, by augmentation, be repeated in half notes or, by diminution, in eighth notes. 'Chiasmus,' borrowed from the terminology of rhetoric, 'designates an a-b-b-a arrangement.'" Moynihan says, "In these lines from 'It Is the Sinner's Dust-Tongued Bell,'
Time's coral saint and the salt grief drown a foul sepulchre
And a whirlpool drives the prayerwheel,
the two reversals in sound (consonantal chiasmus) reinforce the reversal in thought, the paradox. ("Sepulchre" merely repeats the s-k complex). [...] Two examples of phonemic chiasmus from "In Country Sleep"--"Illumination of music" reversed four lines later as "music of elements," and "lulled black-backed/ Gull"--..."
William T. Moynihan, The Craft and Art of Dylan Thomas, 1966