@Phil Smith You're correct about the movement of a C major to a Cdim. So, absolutely, you can drop the 3rd and 5th in CEG by a half step. C Eb Gb I teach it slightly differently, but all roads end up the same place. Where I might not follow, is where you mentioned C dim being in the key of Eb. Eb is - Eb Fm Gm Ab Bb Cm Do Robbies post and my elaboration of it, talks about a Diminished 7 chord, which is different than a Diminished triad. The diminished seventh, is essentially a diminished triad, but with a double-flatted (bb) seventh. R b3 b5 bb7 The detailed breakdown is interesting and probably more meaningful for those that know the notes of all chords. For those that dont, cool, no problem. So that takeaway becomes, any Diminished 7 chord, if you lower any one note a half step, becomes a specific Dom7 chord. In my breakdown, I tried to bring people along, by essentially “giving the answers of the test” so they could follow. “What notes are in a B Dim 7?” - “I don’t know…” “No problem, heres the answer: BDFAb. Now we can continue” But, this feedback is useful because what we share is only useful to the extent that we can teach it and have it be clear. So this makes Me want to go back and evaluate my presentation as a teacher, as to how better not to confuse/lose others. Because if someone doesn't understand it, as a teacher, I believe that it falls upon Me to do a better job refining and explaining. You’ll eventually hear it from Me, because I say it with a lot of passion, “I don’t want anyone left behind”. One of the things that I am always trying to do is reach as many people as possible by simplifying things as much as possible without ever becoming condescending, in doing so, by simply watering things down.