I was working on "For the Good Times" by KK and there's a beautiful, complex chord just before the words "For the good times" at the end of the chorus. In my book it's A7b9. The flattened ninth is what really "makes" the chord. But it's a b*tch to play it accurately and get out of it cleanly. On my Willy (rofl) you have to put the bar at the 10th fret, pedal A&B, E's lowered, and then flatten that ninth on the A by using LKV to lower that fifth string one semitone. Ok, you can just half-pedal the A instead, but either way the chord is no picnic.
I did some thinking: where else can I find that note instead? Surprise surprise, there's a much easier way to get that same chord: bar at the fifth fret, raise E's. That's A#dim. Musically, it seems to be identical. So much easier and quicker to find, because diminished chords recur every 3 frets up and down. So if you are around the 10th fret, you can use 8F or 11F. 8F sounds good cus you can slide back into the D on 10. Choice of strings? Avoid 7 & 2, ensure 4 or 8 is included.
So A7b9= A#dim, B7b9= Cdim, etc, always. I don't see this on any of the music theory websites I go to, when I'm looking for chord synonyms. Is this the best kept secret or am I not looking properly? Either way, I'm chuffed to discover it. Yeah yeah, smartypants

etc, but I had to share.
Will