Stay connected, stay informed, and remain healthy with the Apple Watch Series 4. This Apple Watch features a 44mm curved watch face with an LTPO OLED Retina display and digital crown. The crown is how you navigate through watchOS 5 to view your apps and content. The crown has haptic feedback, giving each turn a mechanical click-like feel, and an integrated titanium electrode lets you use the ECG app. On the back, the Apple Watch features an optical heart sensor to quickly check your heart rate, and an electric heart sensor that works with the crown to generate an ECG (electrocardiogram).