“Deep into that darkness peering, long I stood there wondering, fearing, Doubting, dreaming dreams no mortal ever dared to dream before.” – The Raven, by Edgar Allan Poe.
An alliteration is the use of a series of three or more words that start one or two letters that are the same. Some invented examples could be: “Sally the slithering snake, snuck secretly towards the stables”.
There are also many tongue twisters composed by alliterations, like: “She sells sea-shells down by the sea-shore”; or “Peter Piper Picked a Peck of Pickled Peppers”.