Rule R3 allows unstressed function words, including prefixes, to appear before the foot as extrametrical constituents (see D1b and D1d). Such constituents add to the complexity of the verse, however, and their employment must not make it too difficult to recover the underlying two-word pattern at the speed of performance (see P2). Extrametricality is most problematic at the beginning of the verse, where a function word normally signals the presence of a light foot. If an extrametrical constituent in anacrusis is mistaken for a light foot, the first alliteration of the verse will be assumed to mark the beginning of the second foot, and the verse will appear to consist of three feet. If a light foot is mistaken for an anacrusis, on the other hand, the first alliteration will be assumed to mark the beginning of the first foot, and the verse will appear to consist of one foot. Verses most naturally analysed as one or three feet would impose a heavy analytical burden on the audience. Occasional variants of this kind might be scanned successfully on a second try, but a high frequency of such variants would obviously be intolerable. The Beowulf pott, who needed to employ some anacruses, facilitated intuitive scansion by imposing special constraints on light feet. Linguistic change in Old Norse made anacrusis unnecessary, and the poets seem to have abandoned it at the earliest opportunity. Since constraints on light feet were no longer required in fornyrðislag, new possibilities of verse construction arose.