And if you've ever plunked down in bed or on the couch to read something, you've probably discovered that the screen will automatically rotate between portrait and landscape orientations depending on how you hold your phone or tablet. This can be useful, or it can be a huge hassle -- especially if you move around a lot and don't want the screen constantly shifting back and forth.
Step one: Run the app, then load any book.
Step two: Tap the lower right corner of the screen. You should see a little padlock icon.
Step three: While it's still visible, tap that icon to toggle between locked and unlocked settings. When it shows locked, the screen will not rotate from whichever orientation it's currently in. When it shows unlocked, the screen will rotate when you tilt your device.
I should note that if you've disabled auto-rotate in your device's operating system, setting the Kindle app to "unlock" will have no effect; the global settings override it. So if you do want screen rotation, you need to make sure it's enabled in the OS first.
What's more, this toggle appears nowhere in the Kindle app's settings, so I don't blame you if you've been tearing your hair out trying to find it. Amazon hid it in plain sight.