McLuhan has stated that all technology that we create is an extension of some human sense.  Each new technology introduced to our culture is accompanied by a host of services, and paid for by the ensuing disservices. 

The services and disservices are the hidden ground within which the figure of the artifact resonates.  This hidden ground is the ambient environment we live in, but may not always be consciously aware of. 

We tend to follow a path derived from familiarity with our environment.  Introduction of new technology alters our sense ratio and thereby how experience the world, forcing the abandonment of old mental imprints.  Loss of identity, and violence are often the result.  When new technologies alter the ground continuously, survival demands pattern recognition.

How do we responsd to such rapid environmental changes in our society?  How can we perceive a pattern?

As created by McLuhan and others, tetrads aid in  the understanding how media and technology effect and alter our culture and ourselves by expanding our thoughts beyond the "figure".  Probing technological artifacts with tetrads helps to counteract  "rear-view mirror" thinking.

To create a tetrad, and discover insights into any human technology whatsoever, consider any artifact, and ponder these questions:
Enhance:  What does this medium enhance or amplify?
Obsolete:  What does this medium make obsolete?
Retreival:  What does this artifact retrieve from the past, that was previously obselesced?
Reversal:  What does this artifact reverse into, or, flip into, when pushed to extreme?

As McLuhan explains the enhancing and obsolescent characteristics of the artifact relate to the figure, and the retreival and reversal aspects derive from the ground.  Thus, the services and disservices of any technology live within the tetrad.

Each month the Marshall McLuhan Finnegans Wake Reading Club, explores the services and disservices of various kinds of human artifacts: cell phones, reincarnation, voting, thinking of all the services and disservices we can and exploring the tetrad for that object to further reveal our hidden environment. 

Try it, if you can keep your wits!  It's fun!

Check out this original tetrads sent to us special: 
Survivor show by Eric McLuhan.

For more on tetrads, read:
The Global Village by Marshall McLuhan and Bruce Powers
Electric Language:Understanding the Message by Eric Mcluhan