How do I rotate my screen on an external monitor in OS X

1. Open the system preferences
2. Hold down the command and option keys while clicking the display icon
3. Next locate the screen adjustments on the external monitor and you will notice now there is an option called "Rotation".
4. Choose 90 Degrees and now your monitor is in portrait mode.

*A this can also be a funny prank on someone by doing this to their main monitor!

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