Sequence diagrams, explained without the jargon

A sequence diagram answers one question: who talks to whom, and in what order? It's the clearest way to show how a login, a checkout, or an API call really works — step by step, top to bottom.

The three pieces

• Lifelines — the vertical dashed lines. Each one is a participant: a user, a service, a database.
• Messages — the arrows between lifelines. A solid arrow is a call; a dashed arrow is the reply.
• Activation bars — the thin boxes on a lifeline that show when something is busy doing work.

Type it, don't draw it

You don't have to place a single box. In the Text editor tab, type the conversation and the diagram appears:

User->WebApp: Sign in (email, password)
WebApp->Auth: Verify credentials
Auth-->WebApp: Token
WebApp-->User: Welcome back

Canvas and text stay in sync, so a designer can drag and a developer can type — same diagram, two ways to think.

When to reach for one

A sequence diagram answers one question: who talks to whom, and in what order?

Use a sequence diagram whenever order and timing matter: authentication flows, payment steps, message queues, or explaining a bug's exact path. For structure-over-time questions like “what states can this be in,” a state machine fits better.

Open a sequence diagram and try the example above.