McLuhan's Laws of Media
Marshall McLuhan was constantly challenged to provide a scientific basis for his media observations. Taking on the challenge, he and his son, Eric, reviewed Understanding Media with the intention of doing just that. The result was McLuhan's Laws of Media. McLuhan concluded that four things happen to all media and human artifacts; this phenomenon was inevitable, and they apply universally. Additionally, while some results may take years to make themselves apparent, McLuhan insisted that these things all happened simultaneously.
ENHANCE, REVERSE, RETRIEVE, OBSOLESCE.
Because these things happen simultaneously, the McLuhans settled upon the tetrad to display the interlocking nature of these effects.