Manage role of external display on Mac OS X
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