C7b9

— About this chord

C7♭9 (1-3-5-♭7-♭9) is a tense, dark-flavored altered dominant built from a diminished-seventh shape stacked over the root, giving it a sharp, unresolved bite. A core sound in bebop and swing turnarounds, it drives hard toward tonic resolution, often functioning as V7 in a minor key or a chromatic passing dominant. On guitar it's commonly fingered as a "dim7-over-root" shape, moving easily along the neck since diminished shapes repeat every minor third. Frequently used in ii-V-i cadences and blues codas for extra tension before resolving down a half-step or to the tonic. Also searched as Cdom7b9 or C7 flat 9.