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Most of the websites i have looked at, say that the frequency of middle C note is approximately 261.626 Hz. and that middle C on guitar is at the 5th string 3rd fret. but my tuner says the frequency at the 5th string 3rd fret is 130.81 Hz.
For example, if the A above middle C is 440 Hz, the A an octave above that is 880 Hz (info) . In atonal, twelve tone, or musical set theory a pitch is a specific frequency while a pitch class is all the octaves of a frequency.