if I use a layout such as:

```
<layout>
   <{handle}>
       <reference name="content">
          ... {blocks} ...

```

According to [docs](http://devdocs.magento.com/guides/m1x/magefordev/mage-for-dev-4.html) and [tutorial](http://magebase.com/magento-tutorials/demystifying-magentos-layout-xml-part-1/)
the reference's name attribute points to referencing an existing block so I'm just wondering where is the `content` block defined?

Help much appreciated.