In musical notation and performance, what frequency is commonly referred to as Middle C?

Middle C (C4), which got its name from its position on the 88-key piano keyboard, is 261.6 Hertz when calibrated using the A440 pitch standard for instrument tuning.

The A440 standard, in which the A above middle C (A4) is tuned to 440 Hertz, has been used since 1926 as the American music industry's standard for tuning and calibrating instruments.

While A440 is not the only recognized standard pitch, it is the one most commonly used in modern American music. Other standards include scientific pitch and baroque pitch.

Middle C is the lowest note on a standard orchestral flute, and it is the lowest note in the fixed-do Solfege scale.