duvet vs pillowcase

duvet

noun
  • A quilt or usually flat cloth bag with a filling (traditionally down) and usually an additional washable cover, used instead of blankets; often called a comforter or quilt, especially in US English. 

pillowcase

noun
  • A washable, easily removable cloth cover for pillows. 

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