If A0 is the floor of the piano, C8 is the ceiling. It is key number 88, the very last note on the right of a standard 88-key instrument, the highest pitch the keyboard can reach before it simply runs out of keys. Where the bass end of the piano turns into rumble, this end turns into something bright and glassy, a sound with almost no weight to it at all. Its coordinates are precise: C8 is MIDI note 108, and its fundamental sits at 4186.009 Hz, above four kilohertz.

That is high enough to change the character of the instrument entirely. Up here, near the physical limit of the piano's design, the tone is more sparkle than melody, a short, ringing ping rather than a singing line. The strings are so short and tense that they hold a note only briefly, giving these top keys their characteristic flash and quick decay. It is a register composers use sparingly and deliberately. A single note at C8 can add a point of light to a chord, a glint at the top of a texture, precisely because the ear registers it as brilliance more than as a pitch to hum along with.

Melody is not really its job; brightness is. C8 marks the boundary at the opposite extreme from the piano's lowest string. At one end the instrument dissolves into vibration you can barely hear as pitch; at the other it thins into pure shimmer, a fundamental sitting up above four thousand vibrations a second. Between those two limits lives everything the piano can say, and C8 is the last shining note before the keyboard ends.