This chapter goes through the process of adding a specific form id to the theme registry with the hook_theme() function, which allows the creation of a new function that targets a specific form. Specifically in this chapter, comment_form is added to the theme registry so that we can create a ninesixtyrobots_comment_form() function in order to add a column to the comment form so that the name and subject fields are displayed side-by-side.
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This video covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.