Classic themes have been in use since 2005, and as of 2025, they are still more widely used than block themes. They use PHP files as templates. Some of the most commonly created files are functions.php, header.php, footer.php, single.php, and of course, the required index.php and style.css. The templates usually use The Loop to display content available in the global $wp_query object.
The topic of how to write a good classic theme is outside the scope of this document. Remember, this isn’t a tutorial.