An entity relationship diagram or model describes how sets of entities, such as computer processes or organizational roles, interact with each other. An ER model can also list attributes for a particular entity. The most common shape for an entity is a rectangle, while interactions or actions are diamonds, and attributes are ovals. The three main types of ER diagrams are physical data, logical data and conceptual data models.
Grammatical forms in English correspond to classes of entity relationship model components. Pronouns, for example, refer to an entity, while common nouns are categories of entity types. Likewise, transitive and intransitive verbs translate as relationship and attributes types, respectively. Adjectives are attributes for a specific entity, while adverbs are relationship attributes.
The lines that connect entities, attributes and relationships utilize one of several styles to express cardinality. Bachman lines, for example, use filled circles to indicate one connection, open circles for zero and arrows for more than one connection. The first and third shapes can be combined to indicated a connection that may be one or more, depending on other conditions. Other cardinality line styles, including Martin and Chen, use letters and numbers to express similar concepts to the Bachman style.