" Rule: With single letters and groups of letters that are pronounced as individual letters, be guided by the pronunciation: a B road, a TUC leader; but an A road, an FA Cup match, an SAS unit (assuming the abbreviations are not mentally expanded to their full forms, which would alter the " Answer I have ordered an MRI… Reference "a" Pocket Fowler's Modern English Usage a, an, called the indefinite article (or, by some grammarians, determiner). In origin, a and its by-form an are versions of the Old English an meaning ‘one’. (1) Before all normal words or diphthongs an is required (an actor, an eagle, an illness, an Old Master, an uncle).
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