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how do I controll my Apple TV with the watch? Is it an app I have to download?
After you download the Remote app from Apple, you can then go to the REMOTE settings on your apple TV and tell it to use your watch as a remote.

From there you can use the interface on the watch INSTEAD of the apple tv aluminum remote control.
 
I have used it several times, I like it as another option over using my phone. It works very well.
 
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