Frederick Rebsamen

“Each half-line has two strong stresses. Alliteration occurs only on stressed syllables. The first stress of the second half-line, called the “head-stave,” cannot alliterate with the second stress of that half-line, but it must alliterate with one or both stressed syllables of the first half-line. Recitations of Old English poetry were accompanied in some way by a harplike instrument—indeed, it is called a hearpe in Old English—which may have been used to accentuate stresses, possibly to “fill in” for a missing stress in a defective half-line, but there is no way of knowing just how this was done.”
― Frederick Rebsamen, Beowulf