C# Half Diminished

C# Half Diminished, written C#m7b5, is built from C#, E, G and B: the root, minor 3rd, flat 5th and minor 7th.

Its 3rd and 7th, E and B, are its guide tones: they say what kind of chord it is, so voicings are built around them and the root is often left to the bass.

In the progressions here, C#m7b5 is the iiø7 of the Minor ii-V Progression in B, the viø7 of the Minor i-vi-ii-V Progression in E, the iiiø7 of the Minor ii-V to ii-V-I Progression in A and the viiø7 of the Bird Blues in D.