Stop one mac screen from becoming multiple screens

Mirroring displays the same image on each display.

1. Make sure your additional display is properly connected and powered on.
2. From the Apple (?) menu, choose System Preferences.
3. From the View menu, choose Displays.
4. Click the Arrangement tab.
5. Enable (check) "Mirror Displays" in the bottom-left corner.

In the example below. the blue boxes represent each display that is connected. The white bar at the top of both boxes represents your menu bar and is shown on both displays when they are showing the same information.

When mirroring, your Mac attempts to match the closest resolution between the primary (or built-in) and secondary displays.

The usable resolution is the closest matching resolution for both the primary and secondary display. If you choose a resolution that is not supported on both displays, OS X reverts to usistop one mac screen from becoming multiple screens - lion_close_match_resolution.pngtion.

Extended desktop With extended desktop, your displays are treated as one, continuous desktop.

1. Make sure each display is properly connected and powered on.
2. From the Apple (?) menu, choose System Preferences.
3. From the View menu, choose Displays.
4. Click the Arrangement tab.
5. Disable (uncheck) "Mirror Displays".

In the Displays pane of System Preferences, each blue box represents a display that is currently connected. The size of each box is relative to the current resolution of the display it represents. In the example pictured below, the larger box on the left represents an Apple 27" Thunderbolt Display, while the smaller box on the right is the built-in display of a 13" MacBook Air (Color LCD).
The white bar at the top of the left blue box represents the menu bar you see at the top of your display. Menu bar placement determines which display is treated as the primary display in OS X. To change your primary display, click and drag the white bar from one blue box to another.
You can also drag any of the blue boxes to change a display's represented location. Use this feature to make stop one mac screen from becoming multiple screens - lion_mirror_change_display.pngatch their real world location when navigating your desktop.

Asstop one mac screen from becoming multiple screens - lion_display_max_resolution.pngains its maximum resolution settinstop one mac screen from becoming multiple screens - lion_choose_max_resolution_.pngividual displays showing a continuous desktop.

Press Command-F1 to toggle between video mirroring and extended desktop
Press Command-F2 to detect displays