an- vs un-

an-

prefix
  • Without, lacking. 

  • not; used to make words that have a sense opposite to the word (or stem) to which the prefix is attached. Used with stems that begin with vowels and "h". 

un-

prefix
  • absent; lacking; not; negative 

  • do the opposite of, reverse (a specified action) 

  • contrary to or contrasted against traditional norms; unconventional; alternative 

  • Used to form temporary names of elements (such as unbiunium) whose existence has been predicted, and have not yet been given a trivial name. 

  • deprive of, release from, free from, remove from, extract from 

  • intensifying a verb that already suggests opposition or removal 

  • not 

  • Used to form large numbers as the first in the sequence. 

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