# DMN decision table structure

A decision table has a fixed anatomy: input columns on the left, output columns on the right, one rule per row, and the hit policy in the top-left corner.

Once you can read that layout, every DMN table in the world reads the same way.

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## When it matters

* Reading somebody else's decision table.
* Designing one that a business owner will maintain.

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## The symbols

| **Symbol**     | **Means**                                                  |
| -------------- | ---------------------------------------------------------- |
| Hit policy     | Top-left corner. Decides what happens on multiple matches. |
| Input columns  | Left. One per input, with its type.                        |
| Output columns | Right. One or more results.                                |
| Rules          | One per row, read left to right.                           |
| Annotations    | Right-most column. Notes that do not affect the result.    |

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## A few things that catch people out

* Column order does not affect the result, but it affects readability. Put the most decisive input first.
* Every rule needs a value or a dash in every input column.
* If the table has one input column, you probably wanted a decision tree.

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## Related

[DMN requirement elementsWhat feeds what, above the tables.](https://help.gocapable.com/diagrams/dmn-requirement-elements.html)

[DMN hit policiesWhat happens when two rules match.](https://help.gocapable.com/diagrams/dmn-hit-policies.html)

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

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_Pick the hit policy on purpose._