# Rules of Thumb for Modeling

**In short:** Readable diagrams follow a few habits. Give each diagram a title and a key, name each element's type, label every relationship specifically, and keep color coding consistent. These facts come from C4 guidance.

## Explanation

The facts available here come from the C4 model (see the article "C4 Model"). They cover habits that help people read a diagram they did not draw.

**Give every diagram a title.** The title should state the diagram type and its scope. A reader then knows at once what they are looking at.

**State the type of each element.** Do not leave readers guessing whether a box is a person, a software system, a container or a component. Write the type on the box.

**Label every relationship line.** The label should fit the direction and the intent of the relationship. Make it specific. A vague single word such as "Uses" tells the reader very little.

**Keep colors consistent.** If you color-code elements, use the same codes within one diagram and across all diagrams. A color that means different things on different pages misleads readers.

**Add a key or legend.** Choose notation that explains itself where you can. Even so, put a key on every diagram so the notation is explicit.

The article "Self-Invented Notation Drawback" covers why unexplained custom notation is risky. A key reduces that risk.

## Example

A team draws a diagram of an online shop. The title reads "Container diagram for the Online Shop". Each box names its type, for example "Web Application [Container]". One arrow says "Sends order details to" instead of "Uses". Every database is blue on every diagram. A small key in the corner explains the colors and shapes.

## Trade-offs

The available facts describe the habits but not their costs. Types, labels and a key take extra effort and space on every diagram. The C4 guidance still asks for them.



## Checks

- Every diagram has a title that states its type and scope.
- Every diagram includes a key or legend that explains its notation.
- Every relationship line has a label that matches its direction and intent.
- Relationship labels are specific and avoid vague words like 'Uses'.
- Color coding is the same within each diagram and across diagrams, and each element states its type.

## Related

- [C4 Model](https://arc.fuet.ch/concepts/documentation/c4-model.md)
- [Self-Invented Notation Drawback](https://arc.fuet.ch/concepts/documentation/self-invented-notation-drawback.md)
- [UML Modeling Best Practice](https://arc.fuet.ch/concepts/modeling-uml/uml-modeling-best-practice.md)
- [Color Coding in BPMN Models](https://arc.fuet.ch/concepts/modeling-bpmn/color-coding-in-bpmn-models.md)
- [BPMN Layout Guideline](https://arc.fuet.ch/concepts/modeling-bpmn/bpmn-layout-guideline.md)
- [Guidelines for Documentation](https://arc.fuet.ch/concepts/documentation/guidelines-for-documentation.md)
- [arc42 Architecture Documentation Template](https://arc.fuet.ch/concepts/documentation/arc42-documentation-template.md)

## Sources

- [Notation | C4 model](https://c4model.com/diagrams/notation) — c4model.com, accessed 2026-11-09