WordPress/wp-includes/template-loader.php

<?php
/**
 * Loads the correct template based on the visitor's url
 * @package WordPress
 */
if ( defined('WP_USE_THEMES') && WP_USE_THEMES )
	do_action('template_redirect');

// Process feeds and trackbacks even if not using themes.
if ( is_robots() ) :
	do_action('do_robots');
	return;
elseif ( is_feed() ) :
	do_feed();
	return;
elseif ( is_trackback() ) :
	include( ABSPATH . 'wp-trackback.php' );	
	return;
endif;

if ( defined('WP_USE_THEMES') && WP_USE_THEMES ) :
	$template = false;
	if     ( is_404()            && $template = get_404_template()            ) :
	elseif ( is_search()         && $template = get_search_template()         ) :
	elseif ( is_tax()            && $template = get_taxonomy_template()       ) :
	elseif ( is_home()           && $template = get_home_template()           ) :
	elseif ( is_attachment()     && $template = get_attachment_template()     ) :
		remove_filter('the_content', 'prepend_attachment');
	elseif ( is_single()         && $template = get_single_template()         ) :
	elseif ( is_page()           && $template = get_page_template()           ) :
	elseif ( is_category()       && $template = get_category_template()       ) :
	elseif ( is_tag()            && $template = get_tag_template()            ) :
	elseif ( is_author()         && $template = get_author_template()         ) :
	elseif ( is_date()           && $template = get_date_template()           ) :
	elseif ( is_archive()        && $template = get_archive_template()        ) :
	elseif ( is_comments_popup() && $template = get_comments_popup_template() ) :
	elseif ( is_paged()          && $template = get_paged_template()          ) :
	elseif ( file_exists( TEMPLATEPATH . '/index.php' ) ) :
		$template = TEMPLATEPATH . '/index.php';
	endif;
	if ( $template = apply_filters( 'template_include', $template ) )
		include( $template );
	return;
endif;

?>