Device Orientation
Your Android can be used in either portrait or landscape orientation. Rotate the device to change the orientation for the best view.
Portrait orientation is often better for surfing websites and reading documents (not always, however). Landscape orientation is generally preferred for viewing photographs and watching videos.
If your device screen won’t automatically change when you rotate it, swipe down from the top of your device to open Quick Settings and make sure Auto-rotate 
is set to on.
Android devices detect their orientation in space with built-in motion sensors called accelerometers. The accelerometers can detect movement or tilting in multiple directions. They detect orientation (landscape or portrait) and are used in games and apps. Some games use the device as a steering wheel. Other useful apps turn the device into a level.