> For the complete documentation index, see [llms.txt](https://docs.rainbird.ai/rainbird/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rainbird.ai/rainbird/knowledge-modelling/modelling-features/rblang-reference/xml-structure-and-namespaces.md).

# XML structure and namespaces

Every RBlang document starts with the XML declaration and the root knowledge base element.

```xml
<?xml version="1.0" encoding="utf-8"?>
<rbl:kb xmlns:rbl="http://rbl.io/schema/RBLang">
    <!-- knowledge base content -->
</rbl:kb>
```

### Required structure

* `<?xml version="1.0" encoding="utf-8"?>`
* `<rbl:kb>` as the root element
* `xmlns:rbl="http://rbl.io/schema/RBLang"` on the root element

Use UTF-8 encoding.

{% hint style="warning" %}
The `xmlns:rbl` namespace is required.
{% endhint %}

### Recommended definition order

Define elements in this order:

1. Concepts
2. Relationships
3. Concept instances for `string` concepts
4. Relationship instances and rules

### Core elements

RBlang uses these core elements:

* `<concept>` for concept definitions
* `<rel>` for relationship definitions
* `<concinst>` for string concept instances
* `<relinst>` for facts and rules
* `<condition>` for rule conditions

### Skeleton template

```xml
<?xml version="1.0" encoding="utf-8"?>
<rbl:kb xmlns:rbl="http://rbl.io/schema/RBLang">
    <concept name="Person" type="string"/>
    <concept name="Age" type="number"/>

    <rel name="has age" subject="Person" object="Age"/>

    <concinst name="John Smith" type="Person"/>
    <relinst type="has age" subject="John Smith" object="25"/>
</rbl:kb>
```

### Type naming in RBlang

RBlang uses the type name `truth` for boolean concepts.

```xml
<concept name="Is Eligible" type="truth"/>
```


---

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