The templates in the project (myproject/templates) override the
templates in the app (app_dir/templates). The templates in the app
directories are loaded using the
"django.template.loaders.app_directories.load_template_source" loader,
which is specified by default in the TEMPLATE_LOADERS setting.
Since the templates are in a flat namespace, typically the template
directory has one subdirectory for each app so you could use {%
extends "app_name/template_name.html" %}.
If your app is not reusable, then you may put the templates in
myproject/templates/myapp. If your app will be reused in different
projects, it might be better to provide some default templates with
the app itself (i.e. put the templates inside app_dir/templates).
See
http://docs.djangoproject.com/en/dev/ref/templates/api/#loading-templates
for more info.