head-to-head vs run-in

head-to-head

adj

  • Direct, one-to-one. 

adv

  • Directly, one-to-one. 

  • Facing one another. 

run-in

adj

  • (sometimes comparable) Having been run in to seat the parts. 

  • (not comparable) Having been run in before or behind previous text. 

noun

  • The end-phase of a competition. 

  • An encounter; a scrape or brush, especially one involving trouble or difficulty. 

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