I'm working with get_pages on a site that has a very clean and hierarchical structure.

As part of a template, I wanted to show child pages as part of the template's 'boilerplate'. Reading the Codex documentation, there are two parameters that seem relevant: parent and child_of.

I've been fiddling with them using a simple dump to the body of the page like this but I can't see how the two parameters fundamentally differ...except to offer the risk that they will clash.

For example, if i set child_of to 17 (a valid post) and parent to anything except 17 or -1...I get an empty array. Which makes sense...but...why two parameters? Backward compatibility I can't find when scanning around the docs and code?

Obligatory code used for testing:

$args = array(
            'sort_order' => 'ASC',
            'sort_column' => 'post_title',
            'hierarchical' => 1,
            'exclude' => '',
            'include' => '',
            'meta_key' => '',
            'meta_value' => '',
            'authors' => '',
            'child_of' => 17,
            'parent' => -1,
            'exclude_tree' => '',
            'number' => '',
            'offset' => 0,
            'post_type' => 'page',
            'post_status' => 'publish'
        );
print_r (get_pages($args));

So: Any reason there seem to be two parameters that do the same thing?