Peter Chen, a pioneer of the entity-relationship model, introduced the Chen ERD Notation in 1976. Chen's notation system uses rectangles to represent entities and diamonds to represent relationships, so our above department and employee example would be two rectangles connected by a diamond, with the nature of the relationship shown in the text inside the diamond. Chen's notation uses the letter m to indicate many on one side of the connection to show cardinality. The m stands for any number greater than or equal to one. It uses a 1 to indicate that there is only one.
