an vs em

an

noun
  • The first letter of the Georgian alphabet, ა (Mkhedruli), Ⴀ (Asomtavruli) or ⴀ (Nuskhuri). 

num
  • one 

article
  • Used before one and words with initial u, eu. 

  • Used before a vowel sound. 

  • Used before all consonants. 

  • Used before /h/ in an unstressed syllable. 

  • Used before /h/ in a stressed syllable. 

prep
  • In each; to or for each; per. 

em

noun
  • The name of the Cyrillic script letter М / м. 

  • A unit of measurement equal to the height of the type in use. 

  • The name of the Latin-script letter M. 

intj
  • a form of hesitant speech, or an expression of uncertainty; um; umm; erm 

pron
  • A gender-neutral third-person singular object pronoun, the objective case of ey, equivalent to the singular them and coordinate with him and her. 

How often have the words an and em occurred in a corpus of books? (source: Google Ngram Viewer )