Friday, October 17, 2008
The interval's quality is a bit more complicated. Interval quality describes the specific type of the intervals in addition to the number; intervals can be perfect, major, minor, augmented, or diminished. But not every interval can be of every quality. While all intervals can be augmented or diminished (by adding or subtracting a half step, respectively), only unisons, fourths, fifths and octaves can be perfect; a perfect fourth is five half steps, a perfect fifth is seven, and a perfect unison is zero (since a unison represents the same two notes). Similarly, only second, third, sixth, and seventh intervals can be major or minor; like augmenting or diminishing, this is achieved by adding or subtracting a half step from the intervals.
But be careful. Since major and minor intervals are created by altering the intervals by a half step, augmenting and diminishing works a little differently here. Instead of simply adding or subtracting a half step, augmented intervals (in this case) are a half step more than the interval's major, and diminished intervals are a half step less than the interval's minor. Let's consider third intervals, for example. Major third intervals are four half steps and minor third intervals are three; in a fourth interval, that half step down that creates the minor would create the diminished. But for these intervals we have to go a half step below that, making a diminished third two half steps (which actually creates major second intervals, but that's a story for another time).