How to get iPhone X to display horizontally

Turn the "Portrait Orientation Lock" off. Swipe down from the upper-right corner of your screen to access the Control Center. If the icon that has an arrow circle with a lock in the center is red with white background this means that "Portrait Orientation Lock" is activated. Your screen won't rotate horizontally until you turn off this setting. It is off when the arrow circle with lock center is white with a dark gray/black background. I hope this helps.

Does anything rotate on your iPhone? If it doesn't, from the upper right hand corner of your iPhone swipe down to open Control Center. See the icon with a lock in it. If it is red and with a line through it, tap it and it will open the ability to rotate your screen.

Rotate the screen on your iPhone or iPod touch - Apple Support

I have an app that supposedly supports rotation but it ain’t happening’. Other ideas?

Hooray! That’s the fix! Thank you! I hope that works for you too, Susan W.

As the article states:

”Some apps don’t support rotation...”