Entity relationship diagrams (ERDs) are a graphical representation of entities and their relationships in a database. ERDs were developed in 1976 by Peter Chen and introduced by Charles Bachman. An ERD uses objects, attributes, and relationships to model how data is connected. Relationships can be one-to-one, one-to-many, or many-to-many. Cardinality and modality are also expressed to define maximum numbers and whether relationships are optional or mandatory.