The metrical grammar of Beo 665a cwen to gebeddan is, for that poem, strikingly irregular: the verse is of Sievers Type A with a prepositionand prefix in the first dip and yet displays single - rather than the expected double - alliteration. The Metres of Boethius, a text whose metrical grammar (unlike that of Beowulf) generates a number of similar examples ofthis licence, allows a better understanding of the structure and rhetoric ofthe Beowulf verse. In particular, these analogous exceptions show that theabsence of the expected alliteration accentuates the semantic contrast of thetwo stressed elements of the verse.