else vs if not

else

adj

  • Other; in addition to previously mentioned items. 

adv

  • Otherwise, if not. 

conj

  • For otherwise; or else. 

if not

adv

  • Used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the former. 

How often have the words else and if not occurred in a corpus of books? (source: Google Ngram Viewer )