triad vs trio

triad

noun
  • A grouping of three. 

  • on a CRT display, a group of three neighbouring phosphor dots, coloured green, red, and blue. 

  • A chord consisting of a root tone, the tone two degrees higher, and the tone four degrees higher in a given scale. 

  • A branch of a Chinese underground criminal society, mostly based in Hong Kong. 

  • A word of three syllables. 

trio

noun
  • A group of three people or things. 

  • A piece of music written for three musicians. 

  • A group of three musicians. 

  • Any cocktail made with a spirit, a liqueur, and a creamy ingredient. 

  • A passage in the middle of a minuet, frequently in a different key. 

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