In general, alliteration refers to the repetition of the initial sounds of a series of words. For example, take the sentence: "Sally saw seventeen savages." This sentence repeats the "s" sound four times, resulting in alliteration. Alliteration can also refer to the repetition of the first syllable of a series of words, such as the phrase "he heeded hearing his healer." In this case, the "he" syllable is repeated. An occasional break in the chain of repeated sounds can still be considered a loose alliteration, as is the case with "his."
Consonance refers to the repeated consonant sounds at the beginning, middle or end of a word. Although some claim consonance is not alliteration, many argue that consonance overlaps with alliteration only when the repeated consonant sounds occur at the beginning of the world. An example of consonance can be found in this sentence: "Nimbly, he named the numbers." The repeated "n" and "m" sounds cause both consonance and alliteration.
Assonance refers to the repetition of vowel sounds somewhere within a word. Similarly to consonance, scholars debate whether or not assonance is alliteration. However, it is generally accepted that repetitions of vowel sounds at the beginning of words is both assonance and alliteration. Here's an example of assonance: "All alterations always alter my clothes awfully." The repetition of the soft "a" sound causes alliteration and assonance.
Some alliterations may not be voiced or expressed in speech. In other words, some letters at the beginning of words may be silent and unpronounced, but these letters can still contribute to alliteration. For example, a text could read: "Perry just poked a pink pterodactyl." The repetition of the "p" includes "pterodactyl,"although the "p" in the word is silent. This still counts as alliteration.