A term you'll meet in sound devices in poetry.
Two sound devices that work alongside alliteration but are less often noticed:
Assonance is the repetition of vowel sounds in nearby words: "the rain in Spain stays mainly in the plain" — the long a sound is the connective tissue. The consonants differ; the vowels rhyme without rhyming at the line ends.
Consonance is the repetition of consonant sounds anywhere in the word — beginning, middle, or end: "pitter-patter," "blank and think," "the slithy toves did gyre and gimble." Note that consonance includes alliteration as a special case (initial consonant); when only the end consonants match, it is sometimes called half-rhyme or slant rhyme.
Both devices produce sonic cohesion without the obvious finality of full rhyme. They knit a poem together at the level of sound while leaving the surface less obviously musical. This makes them especially useful in modern poetry, which often wants the binding effect of rhyme without rhyme's traditional sing-song clarity.
Assonance tends to slow a line down — long vowels demand more breath. Consonance, especially with hard consonants, sharpens and accelerates. A poet who knows what they're doing uses both selectively, to control the speed and weight of every line.
Assonance — Tennyson, "The Lotos-Eaters": "The mild-eyed melancholy Lotos-eaters came." The repeated long i and long e sounds draw the line out into the languor the poem describes.
Assonance — Hopkins, "The Windhover": "daylight's dauphin, dapple-dawn-drawn Falcon." Layered vowel music as well as alliteration.
Consonance — Wilfred Owen, "Strange Meeting": Owen famously used pararhyme — full consonance with shifted vowels: "groined / groaned," "hall / hell," "killed / cold." The technique creates a haunting, off-kilter music perfectly suited to the poem's vision of war's dead.
Consonance — Emily Dickinson: Half-rhyme is her signature: "soul / all," "Heaven / given." The slight mismatch unsettles the ear without breaking the rhyme scheme.
Read poetry aloud, slowly. Sound devices are almost invisible on the page but obvious in the mouth. When you notice that a line feels especially cohesive or musically dense, look for repeated vowels (assonance) or repeated consonants beyond the initial ones (consonance). The two devices together explain a great deal of what makes poetry feel made rather than spoken.
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