Here's an answer I posted earlier today to the same question.
Alliteration is often one of the most powerful sound effects in any kind of creative writing, but the precise nature and source of its power are very difficult to describe. Alliteration is almost always used for emphasis of some kind; repeated consonant sounds almost always call strong attention to themselves. Alliteration is especially important in works intended to be spoken aloud, such as Shakespeare’s plays. Yet alliteration can also call attention to itself even when a work is read silently and is heard only by the “inner ear.”
The precise effects of alliteration cannot be described in the abstract. In other words, any attempt to analyze the effectiveness of alliteration depends on a specific and particular context. Take, for example, the famous lines spoken by the witches in the second scene of Shakespeare’s play Macbeth:
Fair is foul, and foul is fair:
Hover through the fog and filthy air.
The alliteration in these lines contributes to their effectiveness in a number of ways, including the following:
- Each of the words emphasized by the alliterated “f” sound is also metrically emphasized. In other words, each of those words is accented. Thus the combination of the repeated “f” and the repeated accents gives those words special stress.
- This double emphasis gives the lines an almost chant-like effect, as if the witches are engaged in some kind of strange ritual. The fact that each of the lines is also shorter than might have been expected (neither has the expected ten syllables of iambic pentameter) also gives these lines special emphasis.
- “Fair” and “foul” are not only alliterated but are also opposites. The witches are denying the typical opposition that exists between these two concepts. They are destroying an important distinction that we usually take for granted. They are implying a world that is topsy-turvy, in which nothing is reliable and everything can easily become its opposite.
- All the “f” words in this passage except “fair” have negative connotations: “foul,” “fog,” “filthy.” All three of these latter words suggest some absence of an ideal. It is almost, then, as if “fair” becomes contaminated and undermined by being associated with these other “f” words, which are unappealing.
- The words here are simple and clear: we all know what “fair” and “foul” mean, and both are monosyllables rather than anything more complicated or difficult to pronounce. However, the witches use these very simple words in paradoxical, complex, and unexpected ways: exactly how (we wonder) can “fair” by “foul” and “foul” be “fair”? The confusion of these opposites suggests something mysterious, mystical, and even unnerving. Our neat categories are being subverted and are being made to bleed into one another. The witches speak with confidence (they don’t say “fair may be foul and foul may be fair”), and they speak of something that already is, not of something that might merely be in the future (they don’t say “fair will be foul and foul will be fair”). They thus speak, in their heavily emphasized way, of a disruption of normality – a disruption that they already know exists. They seem very confident that they know what they are talking about, although their words seems mysterious and unnerving to us.