letter of introduction vs passport

letter of introduction

passport

verb
  • To enable eligibility for a benefit automatically based on eligibility for a different benefit. 

  • Providing the legal requirements to enable or legitimize cross-border economic activity. 

  • To regulate how money given to a local authority must be spent. 

noun
  • An official document normally used for international journeys, which proves the identity and nationality of the person for whom it was issued. 

  • Any document that allows entry or passage. 

  • Something which enables someone to do or achieve something. 

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