In this paper, I analyse the diachronic development of an in the history of English. I propose that in Old English an has the twofold status of a cardinal and an existential operator and has not yet developed into a true indefinite article, whose nature, I argue, is that of an expletive. I show that two distinct stages can be distinguished in Old English, and I propose that the diachronic development of an may be analysed as a typical case of grammaticalization.
