Source

File: wp-includes/template.php

function locate_template( $template_names, $load = false, $require_once = true, $args = array() ) {
	$located = '';
	foreach ( (array) $template_names as $template_name ) {
		if ( ! $template_name ) {
			continue;
		}
		if ( file_exists( STYLESHEETPATH . '/' . $template_name ) ) {
			$located = STYLESHEETPATH . '/' . $template_name;
			break;
		} elseif ( file_exists( TEMPLATEPATH . '/' . $template_name ) ) {
			$located = TEMPLATEPATH . '/' . $template_name;
			break;
		} elseif ( file_exists( ABSPATH . WPINC . '/theme-compat/' . $template_name ) ) {
			$located = ABSPATH . WPINC . '/theme-compat/' . $template_name;
			break;
		}
	}

	if ( $load && '' !== $located ) {
		load_template( $located, $require_once, $args );
	}

	return $located;
}