hairline vs pencil-thin

hairline

adj
  • very thin, as thin as a hair 

noun
  • A fishing line made from hair. 

  • The line along one's forehead where hair starts growing. 

  • A very thin line in writing, drawing, or typography. 

  • A very thin fissure. 

pencil-thin

adj
  • Somewhat cylindrical and slender like a pencil; also, tall and thin. 

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