lead-in vs preparatory

lead-in

noun
  • A short phrase that begins the caption of a photograph. 

  • An introduction; something that leads into the beginning of something. 

  • A region of data at the beginning of a compact disc, holding the table of contents. 

preparatory

adj
  • Of or pertaining to preparation, having the purpose of making something or someone ready, preparative. 

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