a or an — by sound, not by letter

Gist. A stands before a word beginning with a consonant sound, an before a vowel sound. What matters is the pronunciation of the next word, not its first letter. Conditions and exceptions. a unit, a European, a ewe, such a one — initial sound [j] or [w]; an hour, an heir, an honest man, an honour — silent h; letter names and abbreviations go by sound: an MP, an FBI agent, a UN report. Before an unstressed syllable with h-, the 19th-century British norm had an historical novel, an hotel; today a historical prevails. Archaisms: an union, an universal, such an one. Colloquial a before a vowel (a apple) is an error in the text. Examples. a university; an hour; an X-ray; a history, but old an historian. In UD. Lemma and features of a and an are identical, so the annotation does not record the choice of form: this is a check of the text, not of the annotation. Not formalizable in the v0 rule language — it needs word adjacency and pronunciation (e.g. CMUdict: en-cmudict). Sources. Poutsma GLME vol. 3, Ch. XXXI §3 with notes II–V (books/p/po/poutsma-1904-grammar-late-modern-english/text-3.md, pp. 535–536 = book pp. 515–516).

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