Why a,why an

Nidhu Bhusan Das: The other day my niece queried, ‘Uncle, why do we say 'an apple' but 'a mango' and 'a one-rupee' note?’ We know the use but, maybe, do not know the reason thereof. Let me share what I told her. ‘Rupa’, I said, ‘every rule of grammar is based on sound logic and practical need. In this case, phonological realization of language is given importance. Here the sound matters, not the letter which is used for orthographic realization (written form) of language. This is the matter of vowel and consonant sounds. Vowel sounds are of two types – monophthongs (pure vowels) and diphthongs (mixed vowels).Before a monophthong, indefinite article 'an' and before a diphthong and a consonant sound, 'a' is used.’ ‘That I know,’ said Rupa, confident, and asked for the reason. ‘Well,’ I said, ‘vowel sounds have the tendency to embrace the neighboring sounds, be it vowel or consonant. Now, if we say 'a apple' the two neighboring pure vowel sounds represented by the letter 'a' will mix up, and distinct and separate sound to mean 'one' will be missing. When we use 'an' instead, the vowel sound mixes with the neighboring sound, but the presence of the consonant sound represented by the letter 'n' , helps retain the separate identity of the element 'an' as a distinct syllable at least. Now there is no practical need for using 'an' before a diphthong because a diphthong, being a mixed vowel, cannot gobble up the neighboring pure vowel sound. So, we can safely say 'a one' - rupee note.’ Rupa was satisfied.