Dell Venue 8 (7840, Early 2015) - Manual (Page 23)

Using Your Tablet | 23
Screen Orientation
For optimal viewing experience, the screen orientation changes
automatically to portrait or landscape depending on how you hold the
tablet. You can disable automatic screen rotation and lock the screen
in portrait or landscape mode.
NOTE: Some apps may not support automatic screen rotation
and are designed to work only in one orientation.
Locking and unlocking the screen orientation
1 Change the screen to the desired orientation.
2 Swipe down from the top‑right edge of the display to access the
Quick Settings menu.
3 Touch AUTO ROTATE to lock the screen orientation, or touch
ROTATION LOCKED to unlock the screen orientation.