Tetrad of media effects

Generally speaking, a tetrad 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:

What does the medium enhance?
What does the medium make obsolete?
What does the medium retrieve that had been obsolesced earlier?
What does the medium flip into when pushed to extremes?

The laws of the tetrad exist simultaneously, not successively or chronologically, and allow the questioner to explore the "grammar...