How to Set the Display on a Macbook to Mirror

Locate a video-out port on the side of your MacBook. The available ports will depend on your MacBook's model. For example, a MacBook Air has a Thunderbolt port, while a MacBook Pro with Retina display has two Thunderbolt 2 ports and an HDMI port.
Examine the video port on the external display, and then use the appropriate cable or adapter to connect your MacBook to the display. For example, if your display has a DVI port and your MacBook has a Thunderbolt port, use a Mini DisplayPort to DVI Adapter to connect the two devices. In this example, connect the Mini DisplayPort end of the adapter to your MacBook's Thunderbolt port, and then connect the other end to the display's DVI port.
Open the 'Apple' menu, and then select 'System Preferences.'
Click the 'Displays' icon, and then select the 'Arrangements' tab.
Click the 'Mirror Displays' check box in the lower left corner of the window.
Click the red button in the upper left corner to close the window.
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