A popular video calling app from Google for Android and iOS mobile devices. Introduced in 2016 to compete with FaceTime and WhatsApp, Google Duo was quickly downloaded by millions of users and has been praised for its simplicity. Automatically switching between cellular and Wi-Fi, Duo was designed to provide the highest visual quality over slow networks. It also supports end-to-end encryption. Duo's "Knock Knock" feature displays the caller before the recipient answers. See Google Allo and Google Hangouts.