The term musical interval refers to a step up or down in pitch which is specified by the ratio of the frequencies involved. For example, an octave is a music interval defined by the ratio 2:1 regardless of the starting frequency. From 100 Hz to 200 Hz is an octave, as is the interval from 2000 Hz to 4000 Hz. The intervals which are generally the most consonant to the human ear are intervals represented by small integer ratios. Intervals represented by exact integer ratios are said to be Just intervals, and the temperament which keeps all intervals at exact whole number ratios is Just temperament.
| Examples of just musical intervals: | 2:1 | octave |
| 3:2 | fifth | |
| 4:3 | fourth | |
| 5:4 | major third | |
| 6:5 | minor third |
For more information, see here: http://hyperphysics.phy-astr.gsu.edu/hbase/music/mussca.html