Reversal refers to the very general concept that things do not always develop in slow unidirectional increments; instead, sometimes they appear in forms opposite to the form they will eventually take. Using Kenneth Boulding's terminology, McLuhan refers to places where a reversal occurs as "break boundaries."
One of the main reasons McLuhan refers to reversal is that, while electric technology is an acceleration of the mechanical, this doesn't mean that the consequences of it are an increase of the consequences of the mechanical; to the contrary, they often reverse the consequences. Specifically, explosion is reversed to implosion.
- Photography suppressed "the conspicuous consumption of the rich," but movies resulted in "fantasy riches for the poor of the entire globe".
- Roads initially created the concept of cities, but after a break boundary they turned cities into highways and gave highways a city-like character. Also, they created cities as a place of leisure, but after a break boundary they turned cities into places of work and the country into the place of leisure.
- Phonetic writing was a break boundary between tribal and individualist man.
- Print was a break boundary in the history of phonetic writing, resulting in far greater mechanization.