metrical line consisting of four stressed syllables and at least four lesser/un-stressed syllables.
caesura between stresses two and three break line into half-lines
a half-line consists of the pre-caesura line, called the a-verse or on-verse and the post-caesura line, or b-verse or off-verse
ALLITERATION
one or both of the stressed syllables in the a-verse must alliterate with the first stress in the b-verse
the second stress in the b-verse must not alliterate with the first stress
in Old English consonants must strictly alliterate, but all vowels and diphthongs technically alliterate--not sure how that will convey into modern English.