Ever had some weird readings from your Apple Watch, or even wondered if you’re wearing right? Well, don’t worry, AppleInsider found information from Apple on exactly how it wants you to wear the device.

While much of the Apple Watch’s fitness capabilities comes through having internals such as an accelerometer to measure movement or provide fall detection, it also has an array of sensors on the back. If you’ve ever seen someone wearing an Apple Watch too loosely, you’ve seen a bright green light coming from the back. This is the optical heart sensor, which uses photoplethysmography to calculate your heart rate. That back of the Watch, though, must be kept in contact with your skin for that optical and an electrical heart rate sensor to work. The Taptic Engine expects you to be wearing it snugly, and the Wrist Detect feature has to have that skin contact. Apple even provides a diagram recommending the right fit.