Early calculating devices and computers used mechanical digital to analogue converters. This video describes one based on an arrangement of metal bars called a "whiffletree" - also sometimes called a "whippletree." It shows, briefly, the whiffletree used in IBM's revolutionary selectric typewriter and then illustrates the principles of a whiffletree converter by showing the simplest one - one that encodes digital impulses into two bits of information. (This videos is an appendix to Bill Hammack's video about the operation of the Selectric Typewriter.