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This would be the default setting if you don’t do anything to your Display settings. As soon as your Mac senses there are multiple displays you can go into “System Preferences” > “Display“. You’ll be able to see 2 display preferences dialog, one for your prime MacBook display and one for the external monitor. You can use the settings in these dialogs to adjust the screen resolution, color intensity and if you have a monitor that can be swiveled (rotated) you can tell Mac OSX to rotate the display to a certain angle for that particular display.