Tetrad of media effects

Generally speaking, a is any set of four things. In Laws of Media (1988) and The Global Village (1989), published posthumously, Marshall McLuhan summarized his ideas about media in a concise tetrad of media effects. The tetrad is a means of examining the effects on society of any technology/medium (put another way: a means of explaining the social processes underlying the adoption of a technology/medium) by dividing its effects into four categories and displaying them simultaneously. McLuhan designed the tetrad as a pedagogical tool, phrasing his laws as questions with which to consider any medium:

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.

Visually, a tetrad can be depicted as four diamonds forming an X, with the name of a medium in the center. The two diamonds on the left of a tetrad are the Enhancement and Retrieval qualities of the medium, both Figure qualities. The two diamonds on the right of a tetrad are the Obsolescence and Reversal qualities, both Ground qualities.

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