There are no rules you have to follow when playing metal, but many darker sounding bands today try to sound as evil as possible, and part of the recipe for sounding evil is moving in tritones and half steps.
The tritone interval, formerly known as the Devil’s Interval, is found by going up three whole tones, or six frets, from the note you started on. Or you simply jump to the next string and up one fret.
Of course not only Bb will create a tritone interval if you’re riffing out in E – it all depends on the note you play before it. For example, after a G note (3rd fret, low E) a tritone interval would be formed by going up one string and one fret to Db (4th fret, A string).
The half step is the second most evil tool, and is the same as moving up or down one fret on the same string.
Check out this killer lesson from GuitarTricks.com instructor Ben Lindholm
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