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.
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## Base template lookup order
The base template lookup order closely follows that of the template it applies to (e.g. `_default/list.html`).
See [Template Lookup Order](/templates/lookup-order/) for details and examples.
## Define the 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.
{{<codefile=layouts/_default/baseof.html>}}
<!DOCTYPE html>
<html>
<head>
<metacharset="utf-8">
<title>{{ block "title" . }}
<!-- Blocks may include default content. -->
{{ .Site.Title }}
{{ end }}</title>
</head>
<body>
<!-- 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 -->
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.
{{% note %}}
Code that you put outside the block definitions *can* break your layout. This even includes HTML comments. For example:
```go-html-template
<!-- 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)
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 template]:
