Perfect intervals are the unison, fourth, fifth, and octave. The remaining intervals such as the second, third, sixth, and seventh are major intervals. If a major interval is reduced by a semitone we get a minor interval; thus C to E is a major third, but C to Eb is a minor third; C to D is a major second, but C to Db is a minor second; and so on. We have seen that the ratio between the frequencies of the two notes of any octave was 1:2. The ratios between the frequencies of other intervals can also be calculated: for the fifth, 2:3; the fourth, 3:4; the major third, 4:5; the minor third, 5:6; the whole tone, 8:9, and so on. Note that the perfect intervals are characterized by the simpler fractions.