The ‘a’ versus ‘an’ convention is really just to aid pronunciation. The ‘n’ is there to break up the vowel sounds instead of merging them–not to separate actual vowels in written language. That is why “an historic occasion” and “a historic occasion” are both correct–if one does not pronounce the ‘h’, then one must use ‘an’.

So, the usage of “a” or “an” is determined by the sound the word makes rather than the actual first letter. You could say “I have an understanding of grammar” or “I am selling a used book” because the ‘u’ sound in understanding starts with a vowel ‘uhhhh’ noise, but in used, it’s a consonant ‘yu’ sound.