Notation

The C4 model is notation independent, and doesn’t prescribe any particular notation. As a starting point though, a simple notation that works well on whiteboards, paper, sticky notes, index cards and a variety of diagramming tools is as follows.

You can then use colour and shapes to supplement the diagram, either to add additional information or to make the diagram more aesthetically pleasing. Although you may see many example diagrams and tools that make use of blue and grey boxes¹, this isn’t something that is dictated by the C4 model, and you are free to use whatever colours you like!

Elements

Notice that each box includes the element/abstraction type (Person, Software System, Container, Component) - this is a purposeful decision and helps remove any ambiguity about which level of abstraction is being shown.

Relationships

Diagram key/legend

Any notation used should be as self-describing as possible, but all diagrams should have a key/legend to make the notation explicit. This applies to diagrams created with notations such as UML, ArchiMate and SysML too, as not everybody will know the notation being used.

Notation, notation, notation

Although the C4 model is an abstraction-first approach and notation independent, you still need to ensure that your diagram notation makes sense, and that the diagrams are comprehensible. A good way to think about this is to ask yourself whether each diagram can stand alone, and be (mostly) understood without a narrative. You can use this short software architecture diagram review checklist to help.

Here are some general recommendations related to notation.

Diagrams

• Every diagram should have a title describing the diagram type and scope (e.g. “System Context diagram for My Software System”).
• Every diagram should have a key/legend explaining the notation being used (e.g. shapes, colours, border styles, line types, arrow heads, etc).
• Acronyms and abbreviations (business/domain or technology) should be understandable by all audiences, or explained in the diagram key/legend.

Elements

• The type of every element should be explicitly specified (e.g. Person, Software System, Container or Component).
• Every element should have a short description, to provide an “at a glance” view of key responsibilities.
• Every container and component should have a technology explicitly specified.

Relationships

• Every line should represent a unidirectional relationship.
• Every line should be labelled, the label being consistent with the direction and intent of the relationship (e.g. dependency or data flow). Try to be as specific as possible with the label, ideally avoiding single words like, “Uses”.
• Relationships between containers (typically these represent inter-process communication) should have a technology/protocol explicitly labelled.

C4 and UML

Although the example diagrams are created using a “boxes and lines” notation, the core diagrams can be illustrated using UML with the appropriate use of packages, components and stereotypes. The resulting UML diagrams do tend to lack the same degree of descriptive text though, because adding such text isn’t possible (or easy) with some UML tools.

Here are three examples of a system context, container and component diagram for comparison.

System context diagram	Container diagram	Component diagram

C4 and ArchiMate

See C4 Model, Architecture Viewpoint and Archi 4.7 for details of how to create C4 model diagrams with ArchiMate.

Alternative visualisations

Finally, don’t feel that you need to always use a traditional “boxes and arrows” diagram. Although this is usually the default approach, there are other, often interactive, visualisations that can be used to show the same C4 model abstractions in very different ways.

Traditional “boxes and arrows” diagrams are the default approach for documentation and presentations.	A D3.js force-directed graph is a very concise way to visualise larger software architectures, also providing an easy way to explore dependencies.	Ilograph’s interactive diagrams provide a way to selectively zoom in and out, allowing you to explore your entire software architecture model.

• ¹ “C4 = blue and grey boxes” is a common misconception, which is why the example diagrams presented on this website now cycle between blue, green, and red versions!