instrument vs purchase order

instrument

noun
  • A legal document, such as a contract, deed, trust, mortgage, power, indenture, or will. 

  • A device used to produce music. 

  • A means or agency for achieving an effect. 

  • A tool, implement used for manipulation or measurement. 

  • A person used as a mere tool for achieving a goal. 

  • A measuring or displaying device. 

verb
  • To devise, conceive, cook up, plan. 

  • To perform upon an instrument; to prepare for an instrument. 

  • To apply measuring devices. 

purchase order

noun
  • A commercial document issued by a buyer to a seller, indicating the type, quantities and agreed prices for products or services the seller will provide to the buyer. 

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