An interval is the measure of the difference in pitch between two notes. The pitch of a note is accurately described by its frequency. The most harmonious interval between two notes is the 'octave': two notes an octave apart have a frequency ration of 2:1. In other words, the higher note will have twice the frequency of the lower note. The octave is divided into 7 smaller intervals. The number 7, and not 6 or 8, arises from the goal we wish to achieve in dividing up the octave, namely: to achieve the highest number of consonant pairs. In selecting notes that provide consonant pairs, a total of 7 notes proves to be optimal. To see how this arises, we need to further investigate consonance.

Intervals are named according to their position on a 'scale' numbered 1 to 8, 8 being the same note as 1, but one octave higher. Hence an interval can be a: 2^nd, 3^rd, 4^th, 5^th, 6^th, 7^th or octave. As the demands of Western music became more complex, the number of intervals within the octave was expanded, and 5 extra notes were placed between the larger intervals in the above 7-note scale. This gave rise to a 12-note scale, the so-called 'chromatic' scale.

A consonance/dissonance chart of all the intervals, within the chromatic scale, arising from notes above the first, fundamental note, is shown below. The asterisk (*) indicates the 5 extra notes inserted to create the chromatic scale. Four, decreasing degrees of consonance are described: perfect, medium, imperfect and dissonant, based on how closely the two frequencies are related mathematically. See Intervals (in-depth).

Unison     -  perfect consonance   
*              -  dissonant
2^nd              -  dissonant   
*              -  imperfect consonance
3^rd            -  imperfect consonance
4^th            -  medium consonance   
*               -  dissonance
5^th            -  perfect consonance   
*               -  imperfect consonance
6^th            -  imperfect consonance   
*               -  dissonance
7^th            -  dissonance
Octave     -  perfect consonance

The technical names of all 12 intervals may be found in the Interval Name Chart.

Click here to hear the relative consonance and dissonance of two notes, moving apart from the unison.

A visual representation of the consonant intervals can be seen here.

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