what does this symbol mean?

ER diagram symbols

The notation of entity-relationship diagrams: crow’s foot cardinality and Chen’s entities, relationships and attributes, and what each one means.

Test yourself: ER diagram symbols quiz

Crow’s foot cardinality 4 symbols

Chen notation 7 symbols

About eR diagram symbols

An entity-relationship diagram, or ER diagram, is a drawing of the data a database is meant to hold: the kinds of things it keeps records about, the facts it records about each, and how the things relate to one another. The things are entities, the facts are attributes, and the ties between them are relationships. Peter Chen, then at the Massachusetts Institute of Technology, introduced the diagram in a 1976 paper in ACM Transactions on Database Systems, as a tool for database design. An ER diagram describes the data at a high level and contains nothing specific to SQL or to any one kind of database. Each symbol has its own page.

Chen notation and crow’s foot

Most ER diagrams use one of two families of notation. Chen notation draws each Entity as a rectangle and each Relationship as a diamond joined to the entities by lines. Textbooks that follow it, such as Elmasri and Navathe’s, draw each Attribute as an oval, underline a key, and write the cardinality, 1 or N, beside the lines. Crow’s foot notation has no diamonds. A relationship is a line between two boxes, and marks at each end give the cardinality: a bar for one, an open circle for zero and a three-pronged fork for many, so that a bar and a circle together read Zero or one.

Where the crow’s foot comes from

The fork is credited to Gordon Everest, in a 1976 paper on data structure models given at the Fifth Texas Conference on Computing Systems. Everest called it an “inverted arrow”, and later a “fork”; “crow’s foot” is the nickname. A related notation, developed at the British consultancy CACI and promoted by Richard Barker, was adopted by Oracle for its CASE*Method. In Barker’s version a solid or dashed half-line says whether a relationship is mandatory or optional, and entities are rectangles with rounded corners.

Which notation a diagram uses

Many authors have extended Chen’s model in different directions, so the details vary from book to book and tool to tool. Silberschatz, Korth and Sudarshan’s Database System Concepts lists attributes inside the entity rectangle instead of in ovals. Database designers also use UML class diagrams for the same purpose. The same cardinality can be written as a crow’s foot, as a pair of numbers such as (0,N), or as 0..N.

Sources

  1. The Entity-Relationship Model: Toward a Unified View of Data, Peter Pin-Shan Chen, ACM Transactions on Database Systems 1, no. 1 (March 1976): 9–36, Association for Computing Machinery (copy hosted by California State University, Bakersfield), Abstract; p. 9
  2. A Practical Introduction to Databases: 2.2 Entity-relationship diagrams, Christopher Painter-Wakefield, 2022, Runestone Academy, 2.2 Entity-relationship diagrams; 2.2.4
  3. A Practical Introduction to Databases: 2.4 ERD alternatives and variations, Christopher Painter-Wakefield, 2022, Runestone Academy, 2.4.1 Crow’s foot notation; 2.4.2 Common variations
  4. Papers and Publications, Gordon C. Everest, University of Minnesota (Gordon C. Everest’s faculty site), Papers and Publications
  5. Richard Barker (data modeling notations), David C. Hay, 1999, Essential Strategies, Inc., Richard Barker
  6. Database System Concepts, 7th edition, chapter 6 slides: Database Design Using the E-R Model, Abraham Silberschatz, Henry F. Korth and S. Sudarshan, db-book.com, Slide 6.12
  7. Data Modeling Using the Entity-Relationship (ER) Model (chapter 3 slides), Ramez Elmasri and Shamkant B. Navathe, Fundamentals of Database Systems, 2016, Pearson, course CS 448, Purdue University, Slides 3-18 and 3-42

Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research