The affix -ful in words like needful and hopeful originates in the Germanic adjective full, meaning “filled to capacity” (OED, s.v. full, adj.), and has cognates in most languages of its family, both as a free-standing form and as an adjectival suffix. Though the early -ful formations retained the sense of “full of”, its gradual loss of meaning throughout history has substantiated claims about its grammaticalization. Hopper and Traugott (2003: 7) contend that -ful was first part of a phrase (e.g., “a basket full (of eggs)”), later part of a compound (e.g., “a cupful (of water)”), and, finally, a derivational affix (e.g., “needful”), a hypothesis that Wischer (2011: 359) also supports, though with reservations. At any rate, the shift from an independent lexeme into an adjective-deriving suffix on the part of -ful does not only involve changes to syntax and morphology, but also to orthography (Wełna 2000: 45). Today, its affixal status is reinforced by its spelling: though minimal, the difference between -ful and full is substantial. In the same way that grammaticalization often entails phonetic erosion, the process seems to have taken a toll on spelling as well. However, the complexity of English spelling in the Middle and Early Modern periods and the speakers’ erratic relationship towards it make it highly unlikely for users to have intentionally marked this grammaticalization in orthography. The present paper thus aims to analyse the availability of the syntax/morphology-spelling interface from this particular case study, paying closer attention to the aforementioned periods and by means of a corpus-based investigation that draws data from the Helsinki Corpus of English Texts.