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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.
Middle C is on a line below the treble staff. It is probably the most recognisable note in all of music and is the first note most people learn when they start on the piano or keyboard. The line through the note is part of the 'invisible' line which runs between the treble and bass staves when using the Grand Staff. The Spaces in the Treble Clef. The notes in the spaces of the treble staff follow the pattern above D - F - A - C - E - G. You can see that the notes in the middle spaces actually spell F A C E with the D below the staff and G above the staff either side.