instruction vs unwritten rule

instruction

noun
  • An order or command. 

  • An instance of the information or knowledge so furnished. 

  • The act of instructing, teaching, or furnishing with information or knowledge. 

  • A set of directions provided by a manufacturer for the users of a product or service. 

  • A single operation of a processor defined by an instruction set architecture. 

unwritten rule

noun
  • An unspoken rule. 

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