In Laws of Media (1988), published posthumously by his son Eric, 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 (i.e., any 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 and syntax“ of the „language“ of media. McLuhan departs from his mentor Harold Innis in suggesting that a medium „overheats“, or reverses into an opposing form, when taken to its extreme.
Using the example of radio:
- Enhancement (figure): What the medium amplifies or intensifies. Radio amplifies news and music via sound.
- Obsolescence (ground): What the medium drives out of prominence. Radio reduces the importance of print and the visual.
- Retrieval (figure): What the medium recovers which was previously lost. Radio returns the spoken word to the forefront.
- Reversal (ground): What the medium does when pushed to its limits. Acoustic radio flips into audio-visual TV.