# BPMN sequence and message flows

The two line types are the most commonly misused part of BPMN. A sequence flow is control within one pool; a message flow crosses between pools.

Getting this wrong makes a diagram that looks fine and is formally meaningless.

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## When it matters

* Any diagram with more than one pool.
* Processes involving a customer, a supplier or an external system.

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## The symbols

| **Symbol**       | **Means**                                                 |
| ---------------- | --------------------------------------------------------- |
| Sequence flow    | Solid arrow, filled head. Order within a single pool.     |
| Conditional flow | Solid arrow with a small diamond at its source.           |
| Default flow     | Solid arrow with a slash near its source.                 |
| Message flow     | Dashed arrow, open head. Between pools, never within one. |
| Association      | Dotted line. Links an artifact to a flow element.         |

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## A few things that catch people out

* Sequence flow can never cross a pool boundary. If you have drawn one that does, it should be a message flow.
* Message flow can never connect two elements in the same pool.
* The default flow is what happens when no condition matches. Without it, a gateway can stall.

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## Related

[BPMN pools and lanesWho is taking part, and who does what.](https://help.gocapable.com/diagrams/bpmn-pools-and-lanes.html)

[BPMN data objects and storesWhat the process reads and writes.](https://help.gocapable.com/diagrams/bpmn-data-objects-and-stores.html)

[BPMN notation referenceBack to the full symbol reference.](https://help.gocapable.com/diagrams/bpmn-notation-reference.html)

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_Sequence flow stays inside the pool._