The portrait orientation is the default screen orientation for most app designs because it’s the way people naturally hold their devices.
If your app serves for special purposes where the landscape orientation is more appropriate, you can design your apps to use the landscape mode as the default orientation. Games or cameras are good examples where the landscape mode may be used as the default screen orientation.
By designing a screen in landscape orientation, you can take advantage of the wide screen to present specific content to users. However, before using the landscape mode in your app, make sure it serves users’ actual needs. For example, just because your app uses a camera or presents pictures or videos is not a good reason to design your app in landscape orientation, especially if those features are rarely used.
Consider using the landscape orientation when:
Natural transition between portrait and landscape orientations is a standard practice in today's mobile devices. Relative screen layout allows for identical UI component placement in different view modes. However, controls on the screen may have to be adjusted to suit the space allowed in the given orientations. When you design an app that supports both the orientations, ensure that all the features offered in one orientation are also available in the other.
The following figure explains why landscape orientation is ideal for video playback. While the same video can be displayed in both the portrait and landscape orientations, landscape orientation is capable of making the best use of the screen size and displaying larger content.
The Calculator app in the following example switches between the two views depending on the physical orientation of the device. Note that it displays more advanced features only in landscape orientation.