if and only if vs though

if and only if

conj

  • Is equivalent to; implies and is implied by; is true and false in the same cases as. 

  • If, with added intensity 

though

conj

  • Despite the fact that; although. 

adv

  • Used to intensify statements or questions; indeed. 

  • Despite that; however. 

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