clevis vs swallowtail

clevis

noun
  • A U-shaped coupling having holes at each end, through which a bolt is run; used especially to fit attachments to a tractor or other vehicle as it allows a degree of rotation about the bolt. 

swallowtail

noun
  • Anything, such as a burgee, of a similar forked shape. 

  • A type of tailcoat with two long tapering tails.ᵂᵖ 

  • The forked tail of a swallow. 

  • Any of various butterflies of the family Papilionidae, many of which have a long extension to each hindwing. 

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