This page is a companion page to "Sound Effects and the Poetic Line," and should be read in conjunction with it, though whether before or after does not greatly matter.  It can be reached quickly by way of the button below.
     Assonance, consonance, and alliteration are all terms that apply to the repetition of individual sounds.  Assonance is the repetition of vowel sounds; consonance, the repetition of consonant sounds; and alliteration, the repetition of consonant sounds at the beginning of words or of stressed syllables.
     Vowel sounds tend to be comparatively weak and unemphatic,  so assonance is a subtle effect that usually works without calling attention to itself.  Read the line below:

       When the brokers are roaring like beasts on the floor of the Bourse.

The b alliteration is immediately apparent.  The o assonance, also a part of the sound pattern of the line, is much less noticible. 
     It would be convenient if a simple rule could be given for how and why to use certain sounds, but no such general rule exists, though attempts to create one go back to ancient times.  Assonance may be used onomatopoeially, as in a repeated long sound to suggest the squeaking of a small animal or the cry of a bird.  and oo sounds are often used to create a sense of the ghostly, as in Shakespeare's epitaph quoted earlier.  In general, however, the rightness or wrongness of a certain pattern has to depend on the ear of the poet and his readers.
     Often in a piece of verse in which there is no noticeable repetition, we sense that the pattern of vowel sounds contributes greatly to the beauty of the language without being able to say just how.  The lines below from Tennyson's "The Lotus-
Eaters," are an example:

                        'Courage!' he said, and pointed toward the land,
                        'This mounting wave will roll us shoreward soon.'
     In the passage below from Tennyson's "Ulysses," however, assonance does help establish the mood:

                        Matched with an aged wife, I mete and dole
                        Unequal laws unto a savage race,
                        That hoard, and sleep, and feed, and know not me.

In these lines the speaker, Ulysses, is stating his dissatisfaction with his present life, and is about to state his determination to leave on a long journey.  The sound, strongly stressed in the first two lines becomes a pattern in the latter part of the third.  E is a sound that can only be made by thrusting the jaw out and holding it stiffly, a piece of body language that perfectly suits the speaker's mood.
     Consonance, apart from alliteration, is, like assonance, not usually a very emphatic or noticeable feature of a line.  The same basic observations apply to both.  Alliteration is a different matter.  Since the alliterated sounds are also stressed sounds the reader, even if he has an unpracticed ear, does usually notice.

     Alliteration is easy for the poet but, because it does call attention to itself, must be used with care.  The line above  about the brokers takes on a slightly comic quality from its strong alliteration that would make it inappropriate in most contexts.  In the lines below, the alliteration creates utter absurdity:

                                      Round the rampant rugged rocks
                                      Rude and ragged rascals run.
     In Old English  and Old Norse poetry alliteration was a regular part of the meter:  stressed syllables were also alliterated.  Since no one else seems to have any clear rules for what constitutes proper alliteration, I will give the ones from Old Norse, which make quite good sense.  First, alliteration has nothing to do with letters, only sounds.  Wright does not alliterate with Witch, and neither word alliterates with Which.  The first is an sound, the second, a w sound, and the third, an hw sound.  Sh is not an s sound, and does not alliterate with words.  Since s is not an especially strong sound it can be dominated by an explosive sound next to it.  Therefore, sk, sp, and st should only be used to alliterate with the same pair of sounds.  The two th sounds do not alliterate with each other--thin does not alliterate with then.  Much of this, however, is a moot point for either traditional meter or free verse, since there is no requirement that you do alliterate.
     In other poetry, alliteration has no predetermined role; it may be used heavily, subtly, or not at all.  No sound in itself has any exact meaning or even implication, but there are some general rules of thumb for alliteration.  Do not take them too seriously.  Explosive sounds, b, d, k, p, t, are used to give a sense of abruptness or authority.  H suggests breathlessness.  Th is used for a lisping effect.  L produces an effect of lightness.  S suggests smoothness, slyness, or controlled anger.  The impression a sound creates is, of course, largely determined by the context of meaning and sound, and so such statements cannot be used as clear rules.  Here are three examples of alliteration:

                    Not so when swift Camilla scours the plain

                    Of these the false Achitophel was first;
                    A name to all succeeding ages cursed.

                                                          close the serpent sly,
                    Insinuating, wove the Gordian twine.
     In the first of these, by Alexander Pope, the is used to create a sense of quickness and smoothness.  In the second, by John Dryden, the sound hisses with controlled anger.  In the third, by John Milton, the suggests the malice and slyness of the serpent, and of course the serpent's hiss as well.
     No doubt I could think of more, but this covers the basics.  Probably the greatest value to the creative writer is simply the awareness that beyond the words, even the individual sounds he uses in a line of poetry are important both in themselves, and in how they relate to other sounds.   ---Jack Hart
     The link below is to "Imagery:  Metaphor and Simile.     
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