The alphabets of the English script is broadly bifurcated into two divisions namely: Vowels and Consonants. The English script has a total of 26 alphabets among which there are five vowels( a, e, i, o, u) and the rest twenty one are consonants.
English grammar states that words starting with vowels must be preceded by "an" where as words starting with consonants must be preceded with "a".
For example:
An Airplane, An orange, An umbrella, An ice-cream etc.
A boy, A girl, A man, A chocolate etc. <font face="inherit">But there are words which start with a </font>consonant<font face="inherit"> but are preceded by "an" these are exceptions and are so because of their phonetical </font>pronunciation<font face="inherit">.</font>
For example:
An hourglass, An hour
they begin with the consonant "h" but when pronounced they seem to be starting with "o" as the letter "h" is silent and therefore are preceded by "an".