Intervals that produce perfectconsonant notes include unisons, octaves, and
perfect fourths and fifths. Imperfectconsonants include major and minor
thirds and sixths. But basically, if a harmony is consonant, it sounds at least
okay.Not all harmony is easy or pleasant to the ear, though. If you substitute a
G flat for the G, the resulting harmony is dissonant, meaning not so sweet-
sounding (Figure 9-2).Tritone: The devil’s interval................................................................
All the way through the 10th century and beyond, the dissonant augmented
fourth (or flat fifth), also known as a tritone, an example of which is shown in
Figure 9-2, was actually forbidden in music by the Catholic Church. It was
known as the devil’s interval, diabolus in musica, or mi contra fa (all of which
mean roughly the same thing). Play it a few more times to get the sense of
evil that lurks there. Spooky, huh?Nowadays, tritones are used in heavy metal. This is partially thanks to Black
Sabbath’s Tony Iommi’s heavy use of the interval. Iommi mostly started using
it after the tips of two fingers on his fretting hand were cut off in a machine
cutter. His homemade prosthetic wouldn’t form power chords as easily as it
would the tritones. This use of tritones, and the band’s eventual discovery
that they had been playing the devil’s interval all along, led to the tritone
being a standard in metal music well into the present.Without getting too scientific, our ears interpret combinations of pitches as
consonantwhen the pitches and their overtones are either in unison, or are at
least a minor third apart. Pitches that are either closer together than a minor
third or have overtones that are closer than a minor third are perceived of as
dissonant. This is why the interval of a minor second (B and C, for example) is
dissonant, and so is the interval of a tritone, or flat fifth (B and F).&4
4
w
wb
Figure 9-2:
C and G flat
(a tritone)
sound
terrible
together.
Chapter 9: Harmonizing with Melodies 93