The Abominable Flat Nine
A flat nine is the interval of a minor second bumped up an octave. It has about 3.6 times as much bite as a tritone, and is generally avoided in chord harmonization. If you've read any of my write ups on modes, then you should already have a good idea of how to avoid them. However, by avoiding the flat nine, your missing out on a metric crap ton of dissonece. Sometimes, a flat nine can add something that you’re missing out on in a chord progression. They sound awesome when used with appropriate secondary dominants. When used just on their own, it's like an out of tune octave, but worse. For a good idea on what I'm talking about, listen to Deliverance, by Opeth, off of the album of the same name.