Heads up! This subtitles plugin has been deprecated as it lacks Gutenberg support please use the newer WP Subtitle plugin which comes with backwards compatibility with this plugin as well.
Also, not all themes are compatible with the plugins listed in this section. Before using a plugin, make sure it is compatible with the theme by checking the "Recommended Plugins for..." page of your theme's documentation.

Subtitles is a very well coded and simple plugin that allows you to add subtitles to your post title.

Just install and activate it, and at each post/page, you will notice a newly added field for the subtitle, right under the main title of the post/page.