The Johari model is a useful mental model/exercise.
Made (in)famous in 2002 by Donald Rumsfeld’s “there are known knowns…” statement.
Wikipedia entry: https://en.wikipedia.org/wiki/Johari_window
Some examples of definite utility:
http://changingminds.org/disciplines/communication/models/johari_window.htm
And here is a handy conceptual diagram:
