pretzel vs zwieback

pretzel

noun
  • A toasted bread or cracker usually in the shape of a loose knot. 

  • Anything that is knotted, twisted, or tangled. 

verb
  • To bend, twist, or contort. 

zwieback

noun
  • A usually sweetened bread enriched with eggs that is baked and then sliced and toasted until dry and crisp; considered easy to digest and therefore given to the ill and used as a teething food for toddlers. 

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