|The general rule for the use of the indefinite article in modern English is: a is followed by a consonant; an is followed by a vowel. But a problem is caused by certain sounds which display properties of both consonants and vowels - in particular, the ‘semi-vowels’ /w/ and /j/, as heard at the beginning of wet and yet respectively. These are articulated in the same way as vowels (they are really very short versions of /u/ and /i/), but within the structure of a word they take up one of the positions used by consonants, at the beginning of a syllable, as in b-et, s-et, w-et, y-et.
Shakespearean English shows several cases where the modern rule does not operate. There are instances where an is used before /j/:
||There are also several instances where an is used before /h/ in ways that would not be heard today. In modern English, /h/ is used as a consonant, so that we say a horse not an horse. There is a contemporary usage trend where some people use an before h-words beginning with an unstressed first syllable, as in an historic occasion, but the crucial point is that this does not apply to words where the first syllable is stressed; people today do not (yet) say an history book.
However, in Shakespearean English we do find an also used when the h-word is monosyllabic or begins with a stressed syllable: