Harmonizing the major scale For me, this was an important first step to understanding the relationship between chords, scales and key centers. Harmonizing the major scale means building a chord on each note of the scale. As explained in the section on chord construction, the chords are built by going up the scale in steps of two from the note we choose to start on. We'll make 4-voice chords, the standard currency of jazz harmony. For reference, here's the C major scale on the fretboard on the fretboard again: Chord built on C:    C-E-G-B = Cmaj7 Chord built on D: D-F-A-C = Dm7 Chord built on E: E-G-B-D = Em7 Chord built on F: F-A-C-E = Fmaj7 Chord built on G: G-B-D-F = G7 Chord built on A: A-C-E-G = Am7 Chord built on B: B-D-F-A = Bm7b5 The significance of this: because all of these chords were built using only notes from the C major scale, they're all in the key of C. Roman numerals are used to denote the positions of the chords in the scale - for example, Dm7 is the II chord in the key of C, Fmaj7 is the IV chord in the key of C, etc. It's essential to know which key you're in when improvising - and that's not always so easy, because jazz tunes mostly don't stay in one key all the way through like pop and folk tunes do. You have to identify sections that are in a certain key and recognize when the key changes. How to tell? Note that the V is the only 7 chord in the whole sequence. If you see a Dm7 in a tune, it could be the II chord in key of C, but it could also be the VI chord in the key of F or the III chord in the key of Bb. Similarly, Cmaj7 could be the I chord in C, but it could also be the IV chord in the key of F. But if you see G7, it can only be the V chord in the key of C. An example might make that clearer. Here's the first 8 measures of All The Things You Are: The first 7 chord you come to is Eb7 in measure 3. Eb7 is the V chord in the key of Ab. Are the chords around it also compatible with the key of Ab? Yes, they are - Fm7 is the VI in Ab, Bbm7 is the II, Abmaj7 is obviously the I, Dbmaj7 is the IV. But G7 doesn't fit: as we know, this indicates the key of C, and the following Cmaj7 also fits that key. So we have 5 measures in the key of Ab, followed by 3 measures in the key of C. Armed with this knowledge, you could base your improvisation around an Ab major scale fingering for the first 5 measures, then shift up 4 frets to a C major scale fingering. More about that in the section on "Scales and Modes".