pillowcase vs slipcover

pillowcase

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

slipcover

noun
  • A fitted protective or decorative cover that may be slipped off and on a piece of upholstered furniture, usually made of cloth. 

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