Optional Twenty Fifteen WordPress Templates
There are optional WordPress templates in the default Twenty Fifteen theme. Having at least a basic understanding of them is a good idea. The ones listed here give you that good, solid kick in the pants you need to get started with understanding WordPress templates. When you have that task licked, you can move on to learning the rest.
These other (optional) templates include
Comments template (comments.php): The Comments template is required if you plan to host comments on your blog; it provides all the template tags you need to display those comments. The template tag used to call the comments into the template is <?php comments_template(); ?>.
Single Post template (single.php): When your visitors click the title or permalink of a post you’ve published to your blog, they’re taken to that post’s individual page. There, they can read the entire post, and if you have comments enabled, they see the comments form and can leave comments.
Page template (page.php): You can use a Page template for static pages in your WordPress site.
Search Results (search.php): You can use this template to create a custom display of search results on your site. When someone uses the search feature to search your site for specific keywords, this template formats the return of those results.
404 template (404.php): Use this template to create a custom 404 page, which is the page visitors get when the browser can’t find the page requested and returns that ugly 404 Page Cannot Be Found error.
The templates in the preceding list are optional. If these templates don’t exist in your WordPress themes folder, nothing breaks. The Main Index template handles the default display of these items (the single post page, the search results page, and so on). The only exception is the Comments template. If you want to display comments on your site, you must have that template included in your theme.