From CCRMA Wiki
Pendula is a single and two-player network game based on intervals. The single player version is guessing game in which the player first hears and "sees" a target interval and tries to guess that interval by playing it back using the computer keyboard. Played musical intervals are visualized with a harmonogram, which is a plot that shows the relation between the two notes. In the two-player version, players compete against each other to find the target interval in a more challenging way: the last two notes they played are used to form the current interval, so a player should choose what note to trigger depending on what his opponent played.
The idea came when I was making my way out in the bookstore and I bumped over a book called Harmonograph, A Visual Guide to the Mathematics of Music. A Harmonograph is a mechanical device used to visualize the relation between frequencies. It used to be popular in the 19th century in London, when people used to organize parties to watch the amazing figures it could draw. The device consists of pendula (plural of pendulum), attached to a pen mounted on a piece of paper. As a single pendulum oscillates at a specific frequency, the pen would move back and forth along the direction of oscillation, say along the x-axis. Another pendulum oscillating in an orthogonal plane to the first would cause the pen to move along the orthogonal axis, the y-axis. So what you obtain is a superposition of both motions along the x and y axis, which is called a [ Lissajous] curve.