fundamental vs underpinning

fundamental

noun
  • A main or major principle, rule, law, etc. which serves as the foundation or basis of a system; an essential part 

  • The lowest frequency of a periodic waveform. 

  • The lowest partial of a complex tone. 

adj
  • Related to a foundation. base, or basis; serving as a foundation. 

  • essential; extremely important 

underpinning

noun
  • A basis for something. 

  • The lower body or legs. 

  • A support or foundation, especially as a structure of masonry that supports a wall. 

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