 The tetrad first appeared in print in McLuhan's posthumously-published works "Laws of Media" (1988) and "The Global Village" (1989),
The tetrad consists of four questions.The laws of the tetrad exist simultaneously, not successively or chronologically, and allow the questioner to explore the "grammar and syntax" of the "language" of media.McLuhan departs from the media theory of Harold Innis in suggesting that a medium "overheats", or reverses into an opposing form, when taken to its extreme.