Foldable devices are an emerging mobile form factor. Here we introduce you to the features and functionality of the Surface Duo foldable device, explore the device emulator and talk about how to adapt your existing apps to leverage the power of these form factors. We explore best development practices and design challenges creating additional application value for your users on these devices, including code samples and a review of other developer and design resources available. Windows Dev Center -- https://developer.microsoft.com/en-us/windows/?utm_source=developermscom&wt.mc_id=build21_resources_webpage_wwl Microsoft 365 Dev Center -- https://developer.microsoft.com/en-us/microsoft-365/?utm_source=developermscom&wt.mc_id=build21_resources_webpage_wwl Explore Deeper Content and Training -- https://aka.ms/LearnatBuild2021 Microsoft Build 2021