Link.

This is a musical instrument/synthesizer.

You "play" it by clicking to place dots. Each dot represents a note. All dots have a "volume circle" that pulsates. When the volume circle gets bigger, the note gets louder. When the circle gets smaller, the note gets quieter and eventually turns off.

The y-axis determines the frequency of the note (lower towards the top, higher towards the bottom). Horizontal lines represent half steps on a musical scale. Thicker horizontal lines represent a diatonic scale (A major specifically), and green lines represent octaves.

The x-axis determines the modulation speed of the dot/note volume. It is divided up by vertical lines into columns. In the first column (farthest left), the dots will not modulate at all (a steady tone). In the second column they modulate slowly. In the next column they modulate faster, etc.

It is possible to write chord progressions and melodies. I don't know if it's an entirely practical instrument, but fun nonetheless.