Alliteration, Parallelism and Polypoptons
Alliteration is the repetition of the same sounds or of the same kinds of sounds at the beginning of words or in stressed syllables of a phrase. Alliteration developed largely through poetry, in which it more narrowly refers to the repetition of a consonant in any syllables that, according to the poem's meter, are stressed, as in James Thomson's verse "Come…dragging the lazy languid Line along". Another example is, "Peter Piper Picked a Peck of Pickled Peppers".
Consonance (ex: As the wind will bend) is another 'phonetic agreement' akin to alliteration. It refers to the repetition of consonant sounds. Alliteration is a special case of consonance where the repeated consonant sound is at the stressed syllable.
Alliteration may also include the use of different consonants with similar properties, such as alliterating 'z' with 's', as does Tolkien in Sir Gawain and the Green Knight, or as Anglo-Saxon (Old English) poets would alliterate hard/fricative g with soft g (the latter exemplified in some courses as the letter yogh - ȝ - pronounced like the y in yarrow or the j in Jotunheim); this is known as license.
Parallelsim. There is one specialised form of alliteration called Symmetrical Alliteration. That is, alliteration containing parallelism In this case, the phrase must be constituted of two end words both starting with the same letter, and the pairs of outside words getting progressively closer to the centre of the phrase also starting with identical letters. For example, "rust brown blazers rule", "purely and fundamentally for analytical purposes" or "fluoro colour co-ordination forever". Symmetrical alliteration is similar to palindromes in its usage of symmetry.
Polyptoton is the stylistic scheme in which words derived from the same root are repeated (such as "strong" and "strength"). A related stylistic device is antanaclasis, in which the same word is repeated, but each time with a different sense.