Duolingo (/?dju?o?'l??go?/ DEW-oh-LING-goh) is a free language-learning platform that includes a language-learning website and app, as well as a digital language proficiency assessment exam. Duolingo offers all its language courses free of charge. As of November 2016, the language-learning website and app offer 66 different language courses across 23 languages; with 22 additional courses in development. The app is available on iOS, Android and Windows 8 and 10 platforms with about 120 million registered users across the world. The project was started at the end of 2009 in Pittsburgh by Carnegie Mellon University professor Luis von Ahn (creator of reCAPTCHA) and his graduate student Severin Hacker, and then developed along with Antonio Navas, Vicki Cheung, Marcel Uekermann, Brendan Meeder, Hector Villafuerte, and Jose Fuentes. Inspiration for Duolingo came from two places. Luis Von Ahn wanted to create another program that served two purposes in one, what he calls a "twofer". Duolingo does this by teaching its users a foreign language while having them translate simple phrases in documents.