File:Middle C on Fretboard.png

English: The Standard Guitar Fretting effectively turns each of the six strings into a geometric progression of frequencies with a Common Ratio of the Twelfth Root of Two (2^(1/12)). That is, each fret along each string raises the pitch of the note played by the Semitone Ratio (also called the Half Tone), approximately 1.059... times the frequency of the lower note.

The Standard Tuning effectively covers the common musical range of the human voice from Bass through Soprano. It does so by staggering the starting frequency of each string (tuning of the open strings) The sixth string is "Low E", the Fifth String is tuned an Equal Tempered Ma 3rd (2^(5/12)) or Five Semitones higher to A. The ratios between all the strings is Ma 3rd, Ma 3rd, Ma 3rd, Mi 3rd (2^(4/12) or Four Semitones), and Ma 3rd. The purpose of this is to allow the fretting hand to reach and play as many of the most commonly useful chords as possible.

Alternate Tunings (Open G, Open D, etc. will vary the starting frequency for different effects in different forms of music, But this does not change the semitone ratio or perceived interval along each string which is fixed by the frets.

Microtonal, Just and different cultural instruments use different fretting systems which are not addressed here.

Here, Middle C can be played at four locations on the 19 fret guitar shown and also on the 6th sting, 20th fret on instruments with more than 19 frets.