distich vs duad

distich

noun

  • Any couplet. 

  • A couplet, a two-line stanza making complete sense. 

adj

  • Distichous. 

duad

noun

  • A pair or couple. 

  • Dwadasama. 

  • An unordered pair. 

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