Base Templates and Blocks
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.
Base Template Lookup Order
Since Hugo v0.63, the base template lookup order closely follows that of the template it applies to (e.g. _default/list.html).
See Template 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.
<!DOCTYPE html>
<html>
<head>
<meta charset="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 -->
{{ end }}
</body>
</html>
Override the Base Template
From the above base template, you can define a default list template. The default list template will inherit all of the code defined above and can then implement its own "main" block from:
{{ define "main" }}
<h1>Posts</h1>
{{ range .Pages }}
<article>
<h2>{{ .Title }}</h2>
{{ .Content }}
</article>
{{ end }}
{{ end }}
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.
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:
{{ define "title" }}
<!-- This will override the default value set in baseof.html; i.e., "{{.Site.Title}}" in the original example-->
{{ .Title }} – {{ .Site.Title }}
{{ end }}
{{ define "main" }}
<h1>{{ .Title }}</h1>
{{ .Content }}
{{ end }}