To see this effect in action, read out loud this sentence, describing a mother who has discovered the site where her baby was murdered, from Cormac McCarthy's chilling novel, Outer Dark:
And stepping softly with her air of blooded ruin about the glade in a frail agony of grace she trailed her rags through dust and ashes, circling the dead fire, the charred billets and chalk bones, the little calcined ribcage.Notice all the long vowel sounds in that first sentence, in particular the long As. First comes a string of words with that sound, close together: "glade," "frail," "grace," and "trailed." The sentence abandons that sound as it continues, but returns to it for the very last word of the sentence: "ribcage." He's already emphasized this word by ending the sentence with it, and through assonance, in the way the word chimes with those earlier words, he further underlines its importance to this paragraph -- and to the book as a whole.