From the preceding list the following rule is apparent:—
Rule 7. By Changing Enharmonically Either or Both of the Tones of an Interval, a Different Interval is Obtained Which Sounds the Same as the Original Interval.
The distance in semi-tones of all the intervals to an octave is as follows:—
| prime | = | unison | comprises | 1 | letter |
| augmented prime | = 1 | semi-tone | " | 1 | " |
| diminished 2nd | = | unison | " | 2 | letters |
| minor 2nd | = 1 | semi-tone | " | 2 | " |
| major 2nd | = 2 | semi-tones | " | 2 | " |
| augmented 2nd | = 3 | " | " | 2 | " |
| diminished 3rd | = 2 | " | " | 3 | " |
| minor 3rd | = 3 | " | " | 3 | " |
| major 3rd | = 4 | " | " | 3 | " |
| augmented 3rd | = 5 | " | " | 3 | " |
| diminished 4th | = 4 | " | " | 4 | " |
| perfect 4th | = 5 | " | " | 4 | " |
| augmented 4th | = 6 | " | " | 4 | " |
| diminished 5th | = 6 | " | " | 5 | " |
| perfect 5th | = 7 | " | " | 5 | " |
| augmented 5th | = 8 | " | " | 5 | " |
| diminished 6th | = 7 | " | " | 6 | " |
| minor 6th | = 8 | " | " | 6 | " |
| major 6th | = 9 | " | " | 6 | " |
| augmented 6th | = 10 | " | " | 6 | " |
| diminished 7th | = 9 | " | " | 7 | " |
| minor 7th | = 10 | " | " | 7 | " |
| major 7th | = 11 | " | " | 7 | " |
| augmented 7th | = 12 | " | " | 7 | " |
| diminished 8th | = 11 | " | " | 8 | " |
| perfect 8th | = 12 | " | " | 8 | " |
A quicker and better method of determining an interval than by committing to memory the above table is to consider the lower note the tonic of the major scale. If the upper note is in the major scale of the lower note, the interval is normal (major or perfect). After a little practice the number of letters in an interval can be determined at a glance. If the upper note is not in the major scale of the lower note the interval is derivative and is determined by the information heretofore given.
INVERSION OF INTERVALS.
Intervals are said to be inverted when the lower note of the original interval is placed an octave higher, thereby becoming the upper note of the interval thus formed. Example: the inversion of