Inverting an interval means moving the lower note and octave above or moving the top note an octave lower.
When Inverted a:
| Unison | Becomes a | Octave |
| Second | seventh | |
| Third | sixth | |
| Fourth | fifth | |
| Fifth | fourth | |
| Sixth | third | |
| Seventh | second | |
| Octave | unison |
The ordinal number of an interval plus the ordinal number of its inversion always adds up to 9
| Unison | 1 | + | Octave | 8 | = | 9 |
| Second | 2 | + | seventh | 7 | = | 9 |
| Third | 3 | + | sixth | 6 | = | 9 |
| Fourth | 4 | + | fifth | 5 | = | 9 |
| Fifth | 5 | + | fourth | 4 | = | 9 |
| Sixth | 6 | + | third | 3 | = | 9 |
| Seventh | 7 | + | second | 2 | = | 9 |
| Octave | 8 | + | unison | 1 | = | 9 |
Since an interval plus its inversion is an octave the sum of both sets of half steps will always be 12 -
If an interval has 3 half steps the inversion will have 9
| Interval | PLUS | Inversion | adds up to 12 |
| 0 | 12 | ||
| 1 | 11 | ||
| 2 | 10 | ||
| 3 | 9 | ||
| 4 | 8 | ||
| 5 | 7 | ||
| 6 | 6 | ||
| 7 | 5 | ||
| 8 | 4 | ||
| 9 | 3 | ||
| 10 | 2 | ||
| 11 | 1 | ||
| 12 | 0 |
| The inversion of | Perfect | is | Perfect |
| Major | Minor | ||
| Minor | Major | ||
| Augmented | Diminished | ||
| Diminished | Augmented |