# Rule reference
<a name="srgs-rule-reference"></a>

The following table summarizes the various forms of rule reference that are possible within grammar documents. For more information, see [ Rule reference ](https://www.w3.org/TR/speech-grammar/#S2.2) in the *Speech recognition grammar specification version 1* W3C recommendation. 


| Reference type | XML form | Supported | 
| --- | --- | --- | 
| [ 2.2.1 ](https://www.w3.org/TR/speech-grammar/#S2.2.1) Explicit local rule reference | <ruleref uri="\$1rulename"/> | Yes | 
| [ 2.2.2 ](https://www.w3.org/TR/speech-grammar/#S2.2.2) Explicit reference to a named rule of a grammar identified by a [URI](https://www.w3.org/TR/speech-grammar/#term-uri) | <ruleref uri="grammarURI\$1rulename"/> | No | 
| [ 2.2.2 ](https://www.w3.org/TR/speech-grammar/#S2.2.2) Implicit reference to the root rule of a grammar identified by a [URI](https://www.w3.org/TR/speech-grammar/#term-uri) | <ruleref uri="grammarURI"/> | No | 
| [ 2.2.2 ](https://www.w3.org/TR/speech-grammar/#S2.2.2) Explicit reference to a named rule of a grammar identified by a [URI](https://www.w3.org/TR/speech-grammar/#term-uri) with a [media type](https://www.w3.org/TR/speech-grammar/#term-media-type) | <ruleref uri="grammarURI\$1rulename" type="media-type"/> | No | 
| [ 2.2.2 ](https://www.w3.org/TR/speech-grammar/#S2.2.2) Implicit reference to the root rule of a grammar identified by a [URI](https://www.w3.org/TR/speech-grammar/#term-uri) with a [media type](https://www.w3.org/TR/speech-grammar/#term-media-type) | <ruleref uri="grammarURI" type="media-type"/> | No | 
| [ 2.2.3 ](https://www.w3.org/TR/speech-grammar/#S2.2.3) Special rule definitions | `<ruleref special="NULL"/>` `<ruleref special="VOID"/>` `<ruleref special="GARBAGE"/>` | No | 

**Notes**

1. Grammar URI is an external URI. For example, `http://grammar.example.com/world-cities.grxml`.

1. Media type can be:
   + `application/srgs+xml`
   + `text/plain`

**Example**

```
<rule id="city" scope="public">
    <one-of>
        <item>Boston</item>
        <item>Philadelphia</item>
        <item>Fargo</item>
    </one-of>
</rule>

<rule id="state" scope="public">
    <one-of>
        <item>FL</item>
        <item>MA</item>
        <item>NY</item>
    </one-of>
</rule>

<!-- "Boston MA" -> city = Boston, state = MA -->
<rule id="city_state" scope="public">
    <ruleref uri="#city"/> <ruleref uri="#state"/>
</rule>
```