Chapter 5 examines the development of ME ī, which is made more complicated by the fact that ‘ME ī’ covers the reflexes of OE ī/ȳ, i/y in lengthening contexts, iht, yht, eoht, eaht, ēog/ēoh, ēag/ēah, which had all supposedly been raised to [i:] at some stage and therefore fell in with ME /i:/. Consequently, irregular spellings that seem to indicate vowel-shift diphthongisation of /i:/ may in fact not always do so, because they may be back spellings. The chapter assesses a huge number of spellings for the vowels listed above, with a view to determining the chronology of changes in the high-to-mid front vowel area, and to assessing the vowel-shift status of the irregular digraphs. It concludes that diphthongisation seems to be in evidence from at least the late thirteenth century, thus being roughly simultaneous with the raising of ME ē. The chapter also addresses Stockwell & Minkova’s hypothesis that the vocalisation of post-vocalic in lOE or eME may in fact have triggered the shift; their hypothesis is confirmed.