Alliteration is a literary device in which the first consonant is repeated, as well as repeating the same sound in stressed phrases. Tongue twisters are frequently popular examples of alliteration, such as Peter Piper picked a peck of pickled peppers. The world "alliteration" is derived from the Latin word Latira, which means "letters of the alphabet."
One key to alliteration is that it depends, not on the letters, but on the sound. The phrase "not naughty" is an example of alliteration, but "cigarette case" is not, since the "c" sound in "cigarette" is soft, while "case" begins with a hard c. Poets, novelists and even public speakers have employed alliteration in order to provide emphasis to their words. James Joyce's "The Dead" contains the following example of alliteration: "His soul swooned slowly as he heard the snow falling faintly through the universe and faintly falling, like the descent of their last end, upon all the living and the dead." Notice that Joyce employs alliteration in both "s" sounds as "f" sounds.
Even Dr. Martin Luther King, Jr's famous "I Have a Dream Speech" contains alliteration, specifically when he envisions as future where his children will "not be judged by the color of their skin but by the content of their character." As a rhetorical device, alliteration provides texture to the words, catching an audience's ear and making a line more memorable.