The prefix "tri" refers of course to a group of three, as in trinity, tricyle, triathelon and so forth. A musical tritone is an interval of three whole tones (whole steps) or six half tones (half steps). Simply put, a tritone is an augmented fourth or diminished fifth, depending on the key. A tritone is typically created by adding an accidental, but it's found naturally on the fourth and seventh degrees of any scale. In a C major scale, for instance, the tritone would be found by playing the F and B simultaneously.
Tritones are significant because of their ability to create a heavy, uncomfortable dissonance. It's so uncomfortable, in fact, that it has been referred to as the devil's interval and was strongly discouraged during the Baroque period -- a time when the pleasing sound of perfect fifths ruled the day. But despite its rather nasty stigma, the tritone has the power to be pleasing and even somewhat consonant when used correctly. Because it's one of the most moody and easily personified intervals, the tritone is frequently used to foreshadow a heavy resolution.
Additionally, the tritone is helpful to jazz musicians who employ a technique called "tritone substitution." This substitution is possible by playing a dominant seventh chord that uses a root a tritone away from the original dominant seventh. Because the interval pitches are the same, the chords become interchangeable, thus giving the musician a bit more freedom to explore possibly melodies and harmonies during an improvisation.
Jazz isn't the only type of modern music to use the tritone, however. Though it was frowned upon in the early ages of western music, dissonance is far more acceptable today; in some cases, it's even what draws an audience to a certain piece of music. The tritone can be found in nearly every area of modern rock and pop, but it's exploited most often in heavy metal music that prides itself on sounding eerie or evil. Metallica and Black Sabbath have used the tritone to wondrous effect, and one of Jimi Hendrix's most famous songs, "Purple Haze," is based almost entirely around the dissonant chord. The well-known 17-minute song "Inna Gadda Di Vida," by psychedelic rockers Iron Butterfly, also used the tritone during an extended keyboard solo as a way to break the glory of a series of perfect fifths. It's a good thing they weren't around during the Baroque period.
