```
on <i>messageName</i> [<i>parameterList</i>]
    <i>statements</i>
end <i>messageName</i>
```
<br>
The<code> on </code>keyword defines a new message handler of the specified name.<br>
<br>
The optional<code> <i>parameterList</i> </code>lets the handler receive values sent along with a message. HyperCard assigns each value to a parameter variable in the<code> <i>parameterList</i></code>.<br>