The variable operation of high vowel deletion in Old English has long been a point of difficulty, both descriptively – a prehistoric form like *hēafudu is attested variably as hēafudu, hēafdu, and hēafod – and theoretically. Recent work, especially by Bermúdez‐Otero (2005b) and Fulk (2010), has indicated that plural forms like hēafudu are most likely original, but accounting for why the medial *u is preserved in this case form, and not in hēafde, the dative singular of the same word, has remained theoretically problematic. These difficulties arise from attempting to describe the prehistoric Old English process of high vowel deletion on the basis of later Old English phonology. At an earlier stage, the nominative‐accusative plural *hēafudu could be exhaustively parsed into two precisely bimoraic feet: *[hēa][.fu.du]. The dative singular historically ended with a long vowel, *hēafudǣ, in which the medial *u could not be accommodated within a bimoraic foot: *[hēa].fu[.dǣ]. High vowel deletion is therefore best characterized as the deletion of unfooted high vowels in early Old English, initially operating while length in unstressed vowels remained contrastive. Both this quantitative system and the preference for precisely bimoraic units receive support from Kaluza's law, an archaic metrical phenomenon in Beowulf which prohibits resolution in secondary metrical ictus if the resulting unit would have more than two moras, and which is sensitive to prehistoric length distinctions. This original system was obscured, linguistically and metrically, in later Old English by the shortening of unstressed long vowels, triggering various morphological reanalyses of the effects of high vowel deletion. A review of these changes suggests that the system of metrical phonology described here provides a more plausible starting point for the reworkings that produced the forms found in later Old English than do alternative accounts such as those of Campbell (1983) or Ringe (2002).