Christmas card vs postcard

Christmas card

noun
  • A greeting card that is decorated in a manner that celebrates Christmas. 

postcard

noun
  • A rectangular piece of thick paper or thin cardboard intended to be written on and mailed without an envelope. In the case of a picture postcard one side carries a picture or photograph. 

verb
  • To send by means of a postcard. 

  • To send a postcard to. 

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