a

The indefinite article 'a' is one of the most frequently used words in English, yet it did not exist as a separate word until the Middle English period.‌​‍​‍​‍​‌​‌​‌​‌​‌​‌​‌​‌​‍​‌​‍​‍​‌​‍​‍​‌​‌​‌​‌​‌​‌​‍​‌​‍​‌​‍​‌​‍ It is the product of phonological erosion: the Old English numeral 'ān' (one) gradually weakened in unstressed positions until it became a mere grammatical particle.

Old English had no indefinite article. Where Modern English says 'a man came,' Old English would say simply 'man cōm' — the absence of a determiner implied indefiniteness. When speakers wished to emphasize that they meant one particular (but unspecified) individual, they used the numeral 'ān' (one): 'ān man cōm' (one man came, a certain man came). Over the course of the 11th and 12th centuries, this emphatic use became routine, and 'ān' — now pronounced in its unstressed form as 'an' — became obligatory before singular count nouns. Before consonant sounds, the final /n/ was dropped, yielding 'a.' The two forms 'a' and 'an' are therefore not separate words but stress variants of the same article, which is itself a stress variant of the numeral 'one.'

The numeral 'one' descends from Old English 'ān,' from Proto-Germanic *ainaz, from PIE *h₂óynos (one, single). This root produced Latin 'ūnus' (one) — whence French 'un,' Spanish 'un/una,' Italian 'un/uno/una' — showing that the grammaticalization of 'one' into an indefinite article occurred independently in the Romance languages through an identical semantic pathway. German 'ein' functions both as the numeral 'one' and as the indefinite article, perfectly illustrating the transitional stage that English passed through centuries ago.

The family of English words descended from 'ān' is surprisingly large. 'One' preserves the full numeral (the /w/ onset in 'one' /wʌn/ developed in certain Middle English dialects). 'Once' is 'one' + an adverbial genitive suffix. 'Only' is 'one-like' (Old English 'ānlīc'). 'Alone' is 'all one' — entirely one, by oneself. 'None' is 'not one' (Old English 'nān,' from 'ne' + 'ān'). 'Any' is from Old English 'ǣnig,' derived from 'ān' with an adjectival suffix. 'Atone' is a late coinage from the phrase 'at one' — to be at one with someone, to reconcile a division. Each of these words carries the ghost of the numeral 'one' inside it.

The process by which numerals become articles is one of the most regular pathways of grammaticalization in human language. It has occurred independently in English, French, Spanish, Italian, Portuguese, German, Dutch, Turkish, Persian, and dozens of other languages. The indefinite article 'a' is thus not merely a quirk of English but an instance of a universal tendency: languages recruit the concept 'one' to mark indefinite reference, and in doing so they wear the numeral down to a shadow of its former phonological self.