The `block` keyword allows you to define the outer shell of your pages' one or more master template(s) and then fill in or override portions as necessary.
Variables are denoted by capitalized text set within `<>`. Note that Hugo's default behavior is for `type` to inherit from `section` unless otherwise specified.
As an example, let's assume your site is using a theme called "mytheme" when rendering the section list for a `posts` section. Hugo picks `layout/section/posts.html` as the template for [rendering the section][]. The `{{define}}` block in this template tells Hugo that the template is an extension of a base template.
The following defines a simple base template at `_default/baseof.html`. As a default template, it is the shell from which all your pages will be rendered unless you specify another `*baseof.html` closer to the beginning of the lookup order.
<!-- Code that all your templates share, like a header -->
{{ block "main" . }}
<!-- The part of the page that begins to differ between templates -->
{{ end }}
{{ block "footer" . }}
<!-- More shared code, perhaps a footer but that can be overridden if need be in -->
{{ end }}
</body>
</html>
{{</code>}}
## Override the Base Template
From the above base template, you can define a [default list template][hugolists]. The default list template will inherit all of the code defined above and can then implement its own `"main"` block from:
This replaces the contents of our (basically empty) "main" block with something useful for the list template. In this case, we didn't define a `"title"` block, so the contents from our base template remain unchanged in lists.
{{% warning %}}
Code that you put outside the block definitions *can* break your layout. This even includes HTML comments. For example:
```
<!-- Seemingly harmless HTML comment..that will break your layout at build -->
{{ define "main" }}
...your code here
{{ end }}
```
[See this thread from the Hugo discussion forums.](https://discourse.gohugo.io/t/baseof-html-block-templates-and-list-types-results-in-empty-pages/5612/6)
{{% /warning %}}
The following shows how you can override both the `"main"` and `"title"` block areas from the base template with code unique to your [default single page template][singletemplate]:
