Block diagram

Not every diagram needs a notation. A block diagram is boxes, arrows and labels, and it is the honest choice when the shapes carry no special meaning.

Draw it as text if it will change, and on a canvas if the layout is the message.

#When to reach for one

Explaining a system to a non-technical audience.
Early design work, before the components are real enough for a notation.
Slides and summaries, where a strict notation gets in the way.

#How to draw it in Capable

Route	Use when
Mermaid or D2	Automatic layout, so a new box does not mean redrawing.
draw.io	Precise placement, grouping and colour.
Excalidraw	When looking provisional is the point.

#A worked example

Boxes, a group and labelled arrows:

The source, which you can paste into a new diagram and edit:

direction: right

source: Source systems {
  crm: CRM
  billing: Billing
}
pipeline: Ingest pipeline
store: Warehouse {
  shape: cylinder
}
bi: Dashboards

source.crm -> pipeline: nightly export
source.billing -> pipeline: hourly
pipeline -> store: load
store -> bi: query

#A few things that catch people out

Label the arrows. Unlabelled arrows are the reason block diagrams get dismissed as vague.
Group related boxes rather than adding more arrows.
If you find yourself inventing shape meanings, switch to a real notation.

UML diagrams

The UML types you can draw, and what to use for each.

C4 model

Context, containers, components, in that order.

Examples

Working diagrams you can copy.

Cheap to change means it stays true.