  Something I posted on a thread at the Hypnos Boards: Dissonance is one of my favourite musical flavours. Mark Hanson, a professional musician and instructor who used to write for Guitarnation, defines it like this: 'Let me explain dissonance. Dissonance can be described as notes whose frequencies are less than "harmonious" to our ears. For instance, playing notes that are one half-step apart (open first-string E played with the fourth fret of the second string, a D#) is considered dissonant by most people. Playing notes that are a major seventh interval apart, 11 frets, is also considered to be quite dissonant; for instance, play the open second string (B) together with the 6th fret of the first string (A#).
However, dissonance is a requisite part of most music that we hear, providing it with "tension" that leads to "release. " Without it, we end up with music that tends to command little attention. ' From a slightly different perspective, here is how a typical definition of dissonance in the Western art music (or classical) stream would go: 'Harmony that brings about a concordant or agreeable combination of notes is termed consonant. When chords do not fit into an accepted pattern of harmony, they are said to be dissonant or unstable. Dissonance is often used to create moments of suspense that later resolve into more pleasing tones of consonance. Dissonance is sometimes not considered harmony by some teachers of music, although most pedagogy classifies harmony as dissonant or consonant.
' I think those oh-so-simple three notes that form the backbone of Black Sabbath's eponymous epic are among the best examples of dissonance being harnessed to make a tune memorable - you have a low G being played, then the octave , and then, where you would tend to expect a 5th or maybe a 3rd - oho - Iommi and Butler send shivers down your spine with that infernal flatted 5th, the devil's own note in Western art music tradition! Much of blues uses this particular interval, the flatted 5th, which is probably part of the reason for the gritty, enduring appeal of the genre.
Also, dissonance is subjective - whther or not we actually can play music, we all have the forms and norms of the musical tradition we grew up with embedded into our minds' ears. To someone largely used to the wesern musical ethos, much of eastern and oriental music may sound dissonnant at first. This is part of the reason certain western artists like to use eastern scales and modes - its a short cut to spice up their sound.
Dissonance can go awry - I think Korn for instance overuse it, to the point where it looses its impact. Guitarists like Kim Thayil, Tom Morello and those people in Sonic Youth are mong my favourite practitioners of dissonance in the rock tradition. A lot of death metal bands use dissonance in interesting ways, as Axe mentioned. The Velvet Undergrounds' Heroin and The Black Angel's Death Song are primers on dissonance in rock. Entire volumes could be written on forays into dissonance by jazz pioneers like Miles Davis and John Coltrane. I often wish certain modern jazz artists would try and get a little of that shit into their playing! Does anyone know anything about the role or use of dissonance in Indan music? 
