Simon Wheatley

Provides a drop-down to select different templates for posts from the post edit screen. The templates are defined similarly to page templates, and will replace single.php for the specified post. This plugin will NOT switch the templates for the different posts in a listing page, e.g. a date or category archive, it will only affect the template used for single posts (i.e. you can choose a template which is not single.php).
Post templates, as far as this plugin is concerned, are configured similarly to <a href="https://codex.wordpress.org/Pages#Creating_Your_Own_Page_Templates" rel="nofollow ugc">page templates</a> in that they have a particular style of PHP comment at the top of them. Each post template must contain the following, or similar, at the top:
<pre><code><?php
/*
Template Name Posts: Snarfer
*/
?>
</code></pre>
First note: Page templates use “Template Name:“, whereas post templates use “Template Name Posts:“.
Second note: You must have the custom post template files in your theme in the same directory/folder as your index.php template file, not in a sub-directory/sub-folder.
<h3>Developers</h3>
If you want to implement the custom post templates on a custom post type, you can use the <code>cpt_post_types</code> filter, here’s an example below of adding the custom post template selector and metabox to the “Movie” and “Actor” custom post types. This code can be added to a plugin or to the <code>functions.php</code> file in your theme.
<pre><code>/**
 * Hooks the WP cpt_post_types filter 
 *
 * @param array $post_types An array of post type names that the templates be used by
 * @return array The array of post type names that the templates be used by
 **/
function my_cpt_post_types( $post_types ) {
 $post_types[] = 'movie';
 $post_types[] = 'actor';
 return $post_types;
}
add_filter( 'cpt_post_types', 'my_cpt_post_types' );
</code></pre>

Custom Post Template

Provides a drop-down to select different templates for posts from the post edit screen. The templates replace single.php for the specified post.

Melissa Cabral

A simple, lightweight plugin that displays useful technical information on pages and posts to aid in developing WordPress Classic themes. Provides quick access to things that are sometimes hidden, like post/page ID, slug, taxonomy terms, and post type slug. Theme Inspector tells you exactly what conditional tags are true on each view, and what template file loaded on each page view.
<ul>
<li>Use In Conjunction with the <a href="https://docs.google.com/drawings/d/1hJ0MpHO3HKBT5KsTpGtc_gDYZ5pi-HyxNcRtmPBBULE" rel="nofollow ugc">WP Template Hierarchy Document</a>.</li>
<li>Theme Inspector is only visible to logged in Administrators.</li>
<li>Appears on the right-side of the Toolbar (admin bar).</li>
</ul>

Theme Inspector

A developer's inspector to illuminate the WordPress Template Hierarchy and help with building WordPress Classic themes.

Custom Post Template vs Theme Inspector