sixpence vs sou

sixpence

noun
  • A former British coin worth sixpence, first minted in 1551. 

sou

noun
  • An old French copper coin equal to one twentieth of a livre or twelve deniers; one sou is to the livre as one shilling is to the pound. 

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