Looking at jazz standards, the chord progressions can be daunting. But most of the chords fall into one of four categories – diatonic 7th chords, secondary dominants and their tritone substitutions, borrowed chords, and diminished passing chords.
Diatonic 7th chords are chords that are found in the key that the song is in. I, ii, iii, IV, V and vi. You can usually improvise in the key of I over these chords. Of course it helps to know the arpeggios and to be able to find the chord tones.
Non-diatonic chords are everything else. If a chord has at least one note in it that is not in the key of I, it is a non-diatonic chord. Secondary dominants are the most common.
Secondary dominants are chords that are V in relation to something other than I. V/ii, V/IV, V/V and V/vi are the most common. In C, those chords would be A7, C7, D7, and E7, respectively. If you can superimpose those arpeggios over the key of I (changing notes that clash) you can usually generate a very useable scale. For example, if you are in C and the chord is A7, all you need to do is change the C to a C# and you have a nice D melodic minor scale which sounds great. If the chord has been tritone subbed (replaced with the dominant 7 chord a b5 away) you can still use that trick. Eb7 superimposed over C major would be Eb F G A Bb C Db, or Eb lydian dominant.
Borrowed Chords are chords that come from the parallel minor scale. In jazz, the most common of these are the minor iv chord and the dominant bVII chord, but the bVI and the bIII are also heard. Another common example of borrowed chords would be the use of a minor ii-V resolving to a major I. For example, Dm7(b5)-G7(b9)-CMaj7. In all these cases using the parallel minor scale works really well. There are other great choices as well, such as harmonic and melodic minor and sometimes altered scales.
Diminished passing chords are diminished chords that are placed in between two other chords. The most common location is between iii and V/V. In C that would be Em7-Ebdim7-Dm7. The Dm7 then usually goes to G7, and resolves to C. It’s important to note that diminished chords with the root a half step lower than the next chord are usually substitutions for secondary dominants. Cmaj7-C#dim7-Dmin7 is not a diminished passing chord but is really a version of A7(b9). Easy ways to navigate diminished passing chords are the diminished arpeggio as well as the whole/half diminished scale (1 2 b3 4 b5 b6 6 7).
The point here is not that all jazz tunes are easy. They are not. The point is that most chord progressions are relatively standard. Chords function in predictable ways. That’s why we call it functional harmony. If you know your major and minor scales and your dominant seven and diminished arpeggios you can navigate a lot of changes. If you learn a lot of tunes you will start seeing the patterns. That doesn’t make you a master of improvisation, it just means you are keeping your wheels on the road most of the time. From there you still need to develop creative ideas, phrasing, tone, a sense of direction, and lots of other wonderful concepts. As a person who is in the middle (I hope) of his musical journey, I can say it’s a really fun ride!