The circle above is one example of the Pythagorean Circle. It is a heuristic diagram for the complex network of musical interval sizes within a chromatic scale that has 12 semitones to the octave and a harmonic ratio for the octave of 2:1. 

Anticlockwise represents descending fifths (or ascending fourths), clockwise represents ascending fifths (or descending fourths). The note names are adopted from the modern music notation system. The zeros indicate perfect fifths (or fourths, depending on the direction of circulation), i.e. zero tempering in the interval. The '1' indicates an interval that is narrower than a perfect fifth (or fourth) by 1 Pythagorean comma. Any numbers can appear round the Circle, but they must total 1. 

In this convention, a positive number specifies the amount in commas by which that perfect fifth is narrowed from its harmonic ratio 3:2, or the amount the fourth is widened from its harmonic ratio 4:3.   

The Circle will be recognized as the Great Circle of Fifths from music theory text books, but it has the tempering figures added between the note names, and some straight lines added inside the Circle.

The intervals do not necessarily have to be arranged as shown here, i.e. the 1 comma shown between G sharp and E flat could be elsewhere instead, with a different distribution of zeros. The note names should properly reflect the position of the comma, so that the comma is always between two notes that in musical grammar, do not constitute a perfect fifth. For example, we could have circulated clockwise from C to C sharp, and anticlockwise from C to A flat, rather than G sharp. The interval between C sharp and A flat would then contain the comma. Circulation always has to be made round two directions, and the interval between the ends of each circulation will contain the comma. Complete circulation is impossible. For example, starting from C, completing the Circle clockwise in perfect fifths, we would finish not at C but at B sharp, using modern tonal theoretical 'grammar'. The note B sharp may be regarded in modern tonal theory as an 'enharmonic equivalent' of C, but this does not mean that acoustically it is the same note. It is the same note on a piano, but only as a consequence of the fact that the piano is tempered so that none of the fifths are truly perfect fifths, in terms of acoustical consonance.

A Pythagorean comma is the interval between (1) an interval of 12 perfect fifths, and (2) an interval of 7 octaves.

Thus its definition by harmonic ratios, is