How do lock the I-pad to keep it in the horizontal position?

Toggle ON screen rotation lock by swiping a finger downward from the upper right corner of the iPad’s screen to reveal the iOS/iPadOS Control Centre.

Look for the button/icon with a padlock symbol inside of a circular arrow.

Tap that icon/button to lock the screen rotation.

Good Luck!

I have searched and I can find no other posts from you, using that username. As you haven't stated what version of iPadOS/iOS your iPad is running, I am basing my reply on mine, which is running iPadOS 15.1. Pull down from the top right corner to reveal the control panel, and then select the rotation lock button.

Italy1952 Said:

"How do lock the I-pad to keep it in the horizontal position?: how do I lock my i-pad to keep the screen in the horizontal position. this is my third submitting this question."

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Go Here: Change or lock the screen orientation on iPad - Apple Support