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To use both extended and mirrored displays simultaneously, turn off
mirroring then in the “Arrangement” page press and hold the ‘cmd’ and
‘alt’ keys and drag and drop the display you wish to mirror over the
primary display (shown with the white bar).
Repeat this with each display you wish to mirror, all others will act as
an extended display.
Changing the Primary Display
You can change which display is the primary display by moving the
Menu Bar between displays. Changing the primary display affects
where new windows appear and the position of the Dock.
