English: Ford circles. A circle rests upon each fraction in lowest terms. The darker circles correspond to the fractions 0/1, 1/1, 1/2, 1/3, 2/3, 1/4, 3/4, 1/5, 2/5, 3/5, 4/5, and the lighter circles represent other fractions with denominators up to 20. Each circle is tangential to the horizontal axis and some neighboring circles. Fractions with the same denominator have circles of the same size.