The A above the middle C normally has a frequency of 440 cycles per second, or Hertz. This means it vibrates 440 times per second. An A one octave higher has a frequency of 880 Hz, or exactly twice as many vibrations per second. The mathematical expression for this ratio is 880:440, or 2:1. The following table shows the mathematical relationship of several intervals (using just intonation), ordered from consonant to dissonant.

Relation | Interval |
---|---|

2:1 | Octave |

3:2 | Fifth |

4:3 | Fourth |

5:4 | Major Third |

6:5 | Minor Third |

9:8 | Major Second |

16:15 | Minor Second |

See Intervals in the Tutorial section.