: How to C#dim7 & half-diminished chord I find this guide really useful until I find it kinda weird and "wrong" to apply it on key like C sharp half-dimished or diminished chords. I find that I have to double-flat C natural

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You seem to have made a mistake somewhere.

C diminished 7th is C Eb Gb Bbb.

C# diminished 7th has all the notes one semitone higher: C# E G Bb

C half diminished 7th is C Eb Gb Bb.

C# half diminished 7th is C# E G B

Note that there are only three different diminished 7th chords (at least, in equal-temperament tuning) and each one can be "spelled" in different ways. The following are all the same notes:

C Eb Gb Bbb

Eb Gb Bbb Dbb

D# F# A C

Gb Bbb Dbb Fbb

F# A C Eb

Starting on Bbb doesn't work in practice, because you need a "triple flat" for the 7th:

Bbb Dbb Fbb Abbb

But starting on A works OK, of course:

A C Eb Gb

These alternative spellings "explain" why diminished 7th chords are very useful for modulating quickly between almost any pair of keys.

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Your problem started when you thought the 7th note (leading note) in C# major is C. It can't be C, as that would have two different notes with the same C name, to be written on the C line or space on the stave.

That leading note is actually B#. Although enharmonic to C, its name here is B#. As in every note of the C# scale is the same as the C scale, but sharpened.

C# diminished then becomes spelled as C# E, G, Bb. The Bb being a tone lower that B#. C# half diminished then becomes C#, E, G, Bnat. The Bnat. being one semitone lower than the leading note, B#.

Other names are available for the dim. chord itself, but the spelling is here for C#o.