Assonance

Assonance is the repetition of the same vowel sound in nearby words whose surrounding consonants differ, producing an internal echo that is heard rather than seen.

The old stone road showed no one at all.

The long o runs through old, stone, road, showed and no. None of the words rhyme, and the repetition is nearly invisible on the page, but it gives the line a low, held sound that a reader feels as atmosphere.

That is the useful property. Alliteration announces itself; assonance works underneath awareness, which makes it the safest of the sound devices to use at length. Vowel colour carries mood fairly reliably — long low vowels slow a sentence and darken it, high short vowels quicken and brighten it — and a writer who chooses the vowels in a closing line is doing something the reader will feel and never identify.

A worked comparison:

• The little ship slipped in quickly. — short i, fast and light.
• The long boat rode low and slow. — long o, heavy and slow.

Both sentences could describe the same arrival.

The failure mode is the same as everywhere in this family. Pushed too far, assonance turns into unintentional near-rhyme, the sentence starts to chime, and the reader hears the writing instead of the scene — the problem catalogued under mirror words.