We go clockwise to up the interval of a fifth, and counter clockwise to down down a fifth. Starting at C, in the upper middle, after going about a full circle, we find ourselves at B#, which ought to be the same as C, but it's not quite the same. Since each fifth has a ratio of 3/2, after going around 12 fifths, we have gone (3/2)12 = 129.746, but we would have hope to go up by a factor of seven octaves, or 128. So we are off by a factor of 1.0136, which is known as the Pythagorean Comma. This is also the frequency ratio 531441:524288 which is (3/2)12 / 27 or the frequency ratio of 12 fifths to seven octaves.