Manage role of external display on macOS

When you attach a second display to your Mac, you can configure it to be used in two ways

  • Multiple Displays: to extend your desktop and work on two monitors.
  • Mirrored Displays: to show the same desktop on all displays, useful for instance when giving a presentation using a beamer.

The role of the display can be changed in System Preferences -> Displays by toggling the checkbox Mirror Displays.