C♯1 / D♭1 Piano Note Frequency

This black key has two names: C♯1 and D♭1. They are the same key and the same frequency on a piano.

These are the lowest keys on the piano. On most pianos each of these notes has a single heavy string wound with copper wire; the winding adds mass so the string can sound a very low pitch at a practical length. The fundamental is weak this low, so much of what you hear is the higher partials, and tuners set these notes slightly flat of the theoretical frequency (stretch tuning).