central city vs principal city

central city

noun

  • Used other than figuratively or idiomatically: see central, city. 

  • Synonym of central business district 

  • The main city of a metropolitan area; particularly one containing multiple cities. 

principal city

noun

  • Used other than figuratively or idiomatically: see principal, city. 

  • Synonym of central city (“the main city of a metropolitan area; particularly one containing multiple cities”). 

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