Alliteration is the literary device of having two closely associated stressed syllables, normally at the beginning of a word, starting either with the same consonant or with a vowel. In poetry, the words would normally be in the same line (or, in some cases, pair of lines), in prose they would simply be close together.
In poetry, alliteration may either be part of the structure of a poem, or used for a particular effect. In the first case, alliterative verse, it is used in accordance with the rules of that particular type of verse, and the distribution of the alliterated syllables would probably have a fixed relation to the half-line. In the second case the use of the device is up to the writer, and the effect can occasionally be ludicrous.
Verse in which the alliteration is structural is usually known simply as alliterative verse. This was the tradition of northern Europe, including England, until the fashion of rhyme spread northward and took over. In Britain it was Chaucer who did most to establish the new convention, but alliterative verse was still being written by his contemporaries.