At the far left of a standard piano, past the growl of the bass, sits the very last note. A0 is key number 1 on an 88-key instrument, the lowest pitch a normal piano can produce, the floor beneath everything else the keyboard can do. Below the middle of the range, notes are already dark; here at the bottom they become something more like weather. Its numbers are exact: A0 is MIDI note 21 and it sounds at 27.5 Hz, a frequency low enough that it sits at the edge of what the ear can parse.

This is the interesting thing about A0. At this depth the fundamental, the base frequency of the note, gets hard to hear as pitch at all, tending to arrive as rumble rather than a clear musical tone. Much of what you perceive as the note's identity actually comes from its overtones, the higher frequencies ringing above the fundamental, which the ear uses to reconstruct which note is being played. The physical machinery behind the sound is heroic.

Producing a tone this low takes the grand's longest string, a great length of wound steel and copper doing the slow, heavy work of vibrating just twenty-seven and a half times a second. You can almost see it move. A0 marks a boundary as much as a note. It is where the piano stops, where pitch begins to dissolve into vibration you feel more than hear, the deep corner of the instrument that composers reach for when they want the ground itself to sound. One key from the very end, and already the music is turning into something closer to a tremor in the room.