Scientific pitch notation is a way of naming any note in the chromatic scale as well as the octave that it's in. The lowest note in the system is C₀, corresponding to a frequency of 16 Hz, which is roughly the lowest note that the human can detect and an octave below C₁. The lowest note on a standard 88-key piano keyboard is A₀ – 9 half-steps (semitones) above C₀. Middle C is C₄.