How to Do a Dual Screen on an iMac (9 Steps)

Determine the type of video connection used with the external monitor you wish to connect to the iMac. The iMac features a Mini DisplayPort on the back of the monitor that supports DVI, VGA and dual-link DVI connections through the use of different adapters. DVI and VGA are two of the most common connections used with computer monitors. The type of connection your external monitor uses should be printed beneath its connection port (see Resources).
Plug in the appropriate Mini DisplayPort adapter for your monitor. The Mini DisplayPort connector is located in the lower-left corner on the back of your iMac to the right of the USB cable and ethernet cable inputs (see Resources).
Turn on the external monitor and the iMac computer.
Connect the external monitor's connection cable to the Mini DisplayPort adapter that you connected to the back of the computer.
Click the 'Finder' icon on the iMac Dock and then click the 'Applications' folder. Click 'System Preferences' to open the 'System Preferences' dialog box. Click 'Displays' beneath the 'Hardware' subheading to bring up the 'Display Settings' dialog box.
Click 'Detect Displays' in the lower-right side of the dialog box to force the iMac to detect the external display. Once the computer detects the external display, select the 'Arrangement' tab. Two boxes will appear in the dialog box, representing your monitors. The box with the white bar across the top of it represents the iMac monitor while the other box represents the external monitor.
Drag the box that represents your external monitor to the appropriate side of the box that represents your iMac monitor. For example, if your external monitor is on the left side of your iMac, drag the box representing the external monitor to the left side of the box representing the iMac monitor.
Place a checkmark next to the 'Mirror Displays' option in the lower-left corner of the 'Arrangement' dialog box if you want to use the external display to mirror the iMac display. Remove the check mark from the 'Mirror Displays' option if you want to use the external display as an extended display. This means that the monitor will act as an extension of your iMac's desktop, letting you drag the mouse cursor from one screen to the other. You can display programs, movies, applications or anything else on the second monitor to free up space on the iMac's main screen.
Close the 'Display Preferences' window to save the changes.
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