I. Alliteration and rhyme.
A poetic line comprises two verses linked by alliteration, i.e. by repetition of initial sounds. Normally a single consonant alliterates with a single consonant, but the sounds represented by sp, st, and sc alliterate only with identical sounds. Any vowel or diphthong alliterates with any other vowel or diphthong, and in fact the poets generally seem to have avoided alliterating identical vowel sounds, an avoidance that Snorri Sturlusson (d. 1241) tells us Old Icelandic poets practised. The first fully stressed syllable in the on-verse (or a-verse, the first half of the line) should alliterate with the first fully stressed syllable in the off-verse (or b-verse)…
Fulk, Robert. "Old English Poetic Form". The Literary Encyclopedia. First published 22 March 2003
[http://www.litencyc.com/php/stopics.php?rec=true&UID=1268, accessed 20 February 2017.]