If a theme does not specify a set of regions, Drupal will assume the default set of regions

The default regions are defined as:

• page.header: Items for the header region.
• page.primary_menu: Items for the primary menu region.
• page.secondary_menu: Items for the secondary menu region.
• page.highlighted: Items for the highlighted content region.
• page.help: Dynamic help text, mostly for admin pages.
• page.content: The main content of the current page.
• page.sidebar_first: Items for the first sidebar.
• page.sidebar_second: Items for the second sidebar.
• page.footer: Items for the footer region.
• page.breadcrumb: Items for the breadcrumb region.