Apple Watch series 5 wrist placement
Can I wear the watch on the inside of my wrist and still get full functionality of the watch?
Apple Watch Series 5, watchOS 6
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Best reply
You should wear the watch on top of your wrist.
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For best results, the back of your Apple Watch needs skin contact for features like Wrist Detect, the Taptic Engine, and the electrical and optical heart sensors. Wearing your Apple Watch with the right fit—not too tight, not too loose, and with room for your skin to breathe—keeps you comfortable and let the sensors do their jobs. You may want to tighten your Apple Watch band for workouts, then loosen it when you’re done. In addition, the sensors will work only if you wear your Apple Watch on the top of your wrist.
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copied from: Wearing your Apple Watch - Apple Support