Class Responsibility Collaboration (CRC) card

Class Responsibility Collaboration (CRC) cards are a brainstorming tool used in the design of object-oriented software. Martin Fowler has described CRC cards as a viable alternative to UML sequence diagram to design the dynamics of object interaction and collaboration.

CRC cards are usually created from index cards. Members of a brainstorming session will write up one CRC card for each relevant class/object of their design. The card is partitioned into three areas:

1. on top of the card, the class name
2. on the left, the responsibilities of the class
3. on the right, the classes with which this class interacts to fulfill its responsibilities.

Using a small card keeps the complexity of the design at a minimum. It focuses the designer on the essentials of the class and prevents her/him from getting into its details and inner workings at a time when such detail is probably counter-productive. It also forces the designer to refrain from giving the class too many responsibilities. Because the cards are portable, they can easily be laid out on a table and re-arranged while discussing a design with other people.