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Optimize your app for one orientation before adding landscape support.
Use landscape orientation to display content differently, not just bigger.
Lots of typing in your app? Support both landscape and portrait keyboards.
Use landscape to offer graphical alternatives for organizing content and info.
Not everyone will try tipping your app on its side. Beware of hiding important content and features behind screen rotation.
Some features may not be appropriate (or physically fit) in landscape view. Avoid disorienting your audience when the layout changes.