STRATUS is an online interactive installation created by Ólafur Arnalds and Halldór Eldjárn. It is an open collaborative composition played by two self-playing pianos. The pianos are located in Harpa Concert Hall in Reykjavík, Iceland.

By going to the webpage you can take part in playing the pianos, by clicking buttons which affect the way the music is played, and watch the pianos respond in real-time. The buttons affect parameters called “Intensity” and “Complexity”. Intensity controls the subdivisions of the endless polyrhythm and Complexity affects the distribution between intervals of notes.

The volume of users connected at each time also affects the composition, if there are many listening, the overall polyphony will increase. This makes the music sound broader and can also create some interesting and unexpected melodies. When a new user joins, she is welcomed by playing a special bass note combination.