Default comments file and how to copy it

I have a custom theme. There is no comments.php file, WordPress is using whatever the default is. I need to modify the comments area, but I want to base my changes off of the default template. Where is the default template and how can I copy it into my own theme’s comments.php?

UPDATE
Sorted it out for myself. The following directory contains default fallbacks if a file is not specified in your custom theme – \wordpress\wp-includes\theme-compat\

I copied the comments.php file from there over to my custom theme and am ready to rock. I was told in a WordPress forum that this file is deprecated though and I should really be building my own.

I got some help from WordPress forum. As of WordPress 3.1 a function called comment_form() creates the submit comment area. You can override default settings by creating an argument list and passing it in as a parameter. Here is a quick example where I change the default header Reply to Comment and then remove the acceptable tags section at bottom of form.

$comments_settings = array(                                                
    'title_reply' => 'Comment',
    'comment_notes_after' => ''
);

comment_form($comments_settings);