duct vs letterbox

duct

noun
  • a tube or elongated cavity (such as a xylem vessel) for conveying water, sap, or air 

  • a vessel for conveying lymph or glandular secretions such as tears or bile 

  • a pipe, tube or canal which carries gas or liquid from one place to another 

  • a layer (as in the atmosphere or the ocean) which occurs under usually abnormal conditions and in which radio or sound waves are confined to a restricted path 

  • an enclosure or channel for electrical cable runs, telephone cables, or other conductors 

verb
  • to enclose in a duct 

  • to channel something (such as a gas) or propagate something (such as radio waves) through a duct or series of ducts 

letterbox

noun
  • A rectangular hole or slot. 

  • A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation. 

  • A delivery point for mail, a box, compartment or slot for this purpose. 

  • A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose. 

verb
  • To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area. 

  • To hunt for letterboxes (containers with logbook and rubber stamp) by following clues. 

adj
  • Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area. 

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