With mobile devices being usable from a landscape or portrait perspective it's important to test both aspects to validate your content is being displayed how you want it!
In iOS the options to rotate the device are under the menu "Hardware." Then click either Rotate Left or Rotate Right depending how you want to change the orientation of the device.
For Android you can use CTRL+F12 to change the orientation of the emulator on windows. If you have a mac I'd recommend using CTRL+FN+F12 to get the same behavior.
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