At the moment the section about Autoloading in the Sf2 vs sf1 comparison chapter is a bit muddled up. It has been partly updated to deal with the changes that came with using composer from Sf2.1 but still references /vendor/bundles (which no longer exists).

Quote from latest version:

As mentioned before, for the autoloader to work, it needs to know that the Sensio
namespace lives in thevendor/bundles directory and that...

As well as this, the explanation doesn't really make it clear how Symfony / composer knows that the namespace ofSensio\Bundle\FrameworkExtraBundle\SensioFrameworkExtraBundle maps tovendor/sensio/framework-extra-bundle/Sensio/Bundle/FrameworkExtraBundle/SensioFrameworkExtraBundle.php.

The last part of the path is easy enough - but how does composer figure out thevendor/sensio/framework-extra-bundle part of the path?

I'm new at this, but I guess that somewhere theSensio namespace is mapped tovendor/sensio/framework-extra-bundle/Sensio - but where is this done? Or have I completely misunderstood?

The filename is cookbook/symfony1.rst and the section header is "Autoloading". See also some brief comments in#2324 .