Recent reconstructions of late Middle English alliterative metre and of the authorial grammar of the relevant texts can be confirmed and developed further by an observation that monosyllabic dips in the second half-line may contain only syllables with either schwa or short final /i/. It follows that stem syllables of words with variable stress (e.g. cómun / común) and almost all suffixes cannot occur in monosyllabic dips. The restriction applies to verse-final dips and (possibly with a lesser regularity) to the monosyllabic dips between the two metrical stresses, but not to the monosyllabic dips in the verse-initial position. The relatively small number of counterexamples makes it difficult to determine whether the lesser regularity in the position between metrical stresses is due to scribal interference, but it is clear that such verses deserve close attention of a metrist or editor. The proposed rule is observed by most but not all longer alliterative poems, with Mum and the Sothsegger providing at least one straightforward exception. Corollaries of the rule include (a) metrical significance of more than two levels of linguistic stress and (b) rhythmical differentiation of the two half-lines, since non-schwa syllables are very frequent in monosyllabic dips in the first half-line. (NY)