**NOTE** If you have no knowledge of theory, then this post will likely make your brain hurt, but if you have a working knowledge of diatonic chord function and the ability to construct or analyze chords, then perhaps this may be of interest. ____________________________ I thought I'd add a quick discussion of two concepts that get discussed here on occasion: diminished chords and chord substitution. I did a search and I didn't find any recent threads on this topic, but my apologies if it has been discussed before. A "passing" chord can be viewed as a chord outside the key that functions to connect two chords in the key. One common example is that of the diminished 7th (dim7) chord that chromatically connects two chords in the key. Keeping it simple in C Major, let's look at an equally basic chord progression: I ii V I. In C Major, we have: | CMaj7 | Dmin7 | G7 | CMaj7 | We can use a passing dim7 chord to flavor this up a bit: | CMaj7 | C#dim7 | Dmin7 | G7 | CMaj7 | The questions that arise are generally these: 1. Why did I choose to locate the chord where I did? and 2. Why does this chord "work?" The most basic answer to question #1 is that chromatic root motion can be an effective tool (but more on how this particular use of the dim7 chord need not necessarily be viewed as chromatic root motion). The answer to question #2 lies in the relationship of the dim7 chord to the Dominant 7th (Dom7) chord. The dim7 chord is constructed from the root with three successive intervals of a minor 3rd. Starting on C# as in my example, doing this gives us this: C# E G Bb (note that mixing sharps and flats is appropriate in this context, because if you go to the key of C# Major to determine which chord tones are altered, you'll find that a dim7 chord consists of the Root (C#), b3 (E), b5 (G), and bb7 (Bb). Now, someone may say that the bb7 should be Cbb, not Bb. OK, go knock yourself out; sure, technically you are correct (but either way, we must mix sharps and flats in this case). The dim7 chord is unique (I believe it's unique in this respect) in that every note in a dim7 chord is also the root of the chord. By this I mean that there are only three dim7 chords and each serves as four "different" dim7 chords. In other words, our C#dim7, which contains C# E G and Bb contains the exact same notes as Edim7, Gdim7, and Bbdim7. The next four dim7 chords are found by moving up a half-step from any of these notes while the last set of four are found by moving up one more half-step. In any case, let's address question #2: The reason why this C#dim7 works so well in this simple progression is because it is nearly identical to the secondary Dominant of the ii chord (Dmin7). A secondary Dominant is a chord that is outside of the key, but functions as a legitimate V7 that resolves to a chord that is in the key. Using our example, the A7 is a secondary Dominant of the Dmin7: | CMaj7 | A7 | Dmin7 | G7 | CMaj7 | Notice that the A7 slots in directly in place of our passing dim7 chord. Why? To answer this question, let's break down the C#dim7 and the A7: C#dim7: C# E G Bb A7: A C# E G The two chords have three of four notes in common. This is always a strong indicator of "Hey, I've got a good chord substitution possibility here." But what about the notes that are not common to the two chords? These notes are the Bb in the C#dim7 and A in the A7. At first glance, this may seem to be a problem; however, it is not. It turns out that the Bb in the C#dim7 chord is the b9 of the A7 chord (more correctly, the b9 (or b2) of the A Major scale from which an A7 chord can be evaluated). The A7 chord is constructed as 1 3 5 b7 of the A Major scale. The reason why the Bb is "beneficial" here is that the V chord when resolving to a minor chord is very often voiced as V7(b9). Thus, our C#dim7 effectively functions as a chord substitution for A7(b9) resolving to a Dmin7. The only note missing is the A natural. Thus, a dim7 chord can be used effectively as a substitution for a V chord that resolves to a min7 chord. The way I generally choose which dim7 chord is to base it on the b9 of the dom7 chord. This brings up my final point, to which I alluded earlier: using my method, if I want to add a substitution for the Dom7 chord in this progression I posted earlier (with the b9 extension added to the A7): | CMaj7 | A7(b9) | Dmin7 | G7 | CMaj7 | I would choose Bbdim7. Let's construct a Bbdim7 chord: Bb Db Fb Abb (or G). Note that these notes are enharmonically equivalent to C# E G Bb. It's up to you whether you want to emphasize the chromatic root motion (C C# D), thus thinking of the chord primarily as a C#dim7, or if you prefer to call it an Edim7, and Gdim7, or a Bbdim7. These are all voicings (or inversions, if you prefer) of the same chord. Sorry, longer than I expected. I hope I made myself clear.