AdDuplex is a cross-promotion network that allows Windows Phone and Windows Store app developers to advertise each other's apps. Developers insert a line of code into their apps to display other developers' ads, and in turn have their own ads displayed by other apps. AdDuplex provides a 100% fill rate for ad space using this method. The service has over 9,000 active published apps and serves over 1 billion ads per month across many countries and platforms.