Duo is a video chat mobile app developed by Google, available on both Android and iOS. It was announced at Google I/O on May 18, 2016 alongside Allo, an instant messaging app. Duo launched in the United States on August 16, 2016 and globally a few days later. Duo quickly climbed to the number one free app on Google Play within two days of its initial release. In an email sent to Android phone manufacturers on October 5, 2016, Google announced that, starting December 1, 2016, Google Duo will replace Hangouts within the suite of Google apps Android phone makers must pre-install on devices, with Hangouts instead becoming an optional app for phone makers to pre-install on their devices.