Q:

WP Post web-pages shows index content

As an amateur WordPress developer i'm trying my luck with my first template, I came across this issue where when ever i access a Post web-page i get the content of the Index.php file. this seems to appear only with Posts web-pages, as this problem does not appear with the Page web-pages. As for now my template contain the following files: 

footer.php , function.php , header.php , index.php , page.php

Am i missing a file that should handle the Posts type? if not what could lead to this kind of problem?

A:

Yes you are missing single.php to handle individual post pages.
Check out https://developer.wordpress.org/themes/basics/template-files/ for some more info on the main template files.

