When creating a child template that 'extends' another template, it is possible to fill in a block while using the default as well.  This is done with the 'parent' keyword.  So lets say that index.html is defined like this:

{% block content %}
<h1;>{{ this.title }}</h1;>
{% endblock content %}

We can now define a child template like this and use what has already been created as a default in 'content':

{% extends 'index.html' %}
{% block content %}
  {% parent %}
  {{ this.content }}
{% endblock content %}

Now the title will be printed because it was defined as default in the parent. After that, the content will be printed.