Alliteration is the repetition of common sounds beginning with a consonant. Alliterative sounds are found at the beginning of the word or in the stressed syllable. Common use of alliteration emphasizes the sound of the initial consonant only. For example, "look before you leap," highlights looking before leaping with the use of the "le" sound of the letter "L" at the beginning of the substantive words. In the Cantos II, by Ezra Pound, the alliterative sounds of the letter "s" and "sh" are utilized to create the musical sound of the sea churning against the rocks.
Consonance, in literature, refers to repetition of ending or internal consonant sounds. For example, in the description "black rock" the ending consonant sounds are the same. Consonance is frequently employed as an ending rhyme scheme in poetry. Rather than emphasize the entire word, as in alliteration, consonance emphasizes the sound at the end of the word.
The inclusion of vowels in the repeated sounds is one difference between consonance and alliteration. Alliteration allows for the inclusion of vowels with consonants to create a repeated sound. Consonance, on the other hand, does not. Consonance includes only consonant sounds. Many uses of consonance include consonant clusters, such as "ch" or "ck."
The Anglo-Saxons and the Welsh used alliteration as the main sound device for poetics. The famous Anglo-Saxon "Beowulf," employs alliteration heavily. Book titles and fictional characters frequently employ alliteration as a mechanism for creating a catchy title or name. Use of alliteration may also be strictly decorative, but was once required as a poetic device.