There are several exercises which will help a person see what is right or wrong with a system.  In this exercise you grab 4 completely blank cards and begin a CrcCard''''''s session with only those cards.  It forces you to think about your design in an abstract way.  You must also decide what it is most important in your system.  This is an exercise to increase your own understanding of the system.  During a normal CrcCard''''''s session where it is the audience who will receive a greater understanding you would use more cards.

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This keeps you healthily below the magical SevenPlusOrMinusTwo limit.  Coincidence?
''Huh? It's not even ''exactly'' SevenPlusOrMinusTwo. It's less of a coincidence than most appearances of the LawOfFives.''