quadriad vs septet

quadriad

noun

  • A grouping of four things. 

  • A government committee of four economic experts. 

septet

noun

  • A set of seven lines. 

  • A group of seven musicians. 

  • A composition for a group of seven musicians. 

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