What is diatonic substitution? How is it used in improvisation?
Diatonic substitution is changing a diatonic chord into another diatonic chord with a similar function. For example, in a C major tonality, you can often reharmonize a melody harmonized with F[maj7] with Dm (or vice versa). These chords share some important notes which makes them functionally similar (both have subdominant character).
Chromatic substitution, on the other hand, is substituting a chromatically altered chord for a chord. For example the tritone substitution, that is, playing a Db7 instead of a G7 is a chromatic substitution. These two (enharmonically) share their third and seventh, which makes them somewhat similar in function (both have dominant character).
You can use substitutions while improvising a "comping" to give the soloist some more possibilities (or a hard time, if he's not skilled enough!). Melodies sound different when played over different chords and it adds variety.
You can also use substitutions when soloing. You can use the scales implied by the substitute chord instead of the actual one. It gives you some more possibilities to add color, variety and tension.
1This might be outside the realm of jazz (which seems to always use 7ths) but would adding or removing the seventh to a chord (from the diatonic scale) be considered a diatonic substitution (as in replacing I with IM7), or does it necessarily involve changing the root (as in replacing IM7 with vim7)? Jun 6, 2014 at 2:26
Honest answer: I have no idea :) Less honest answer: I don't think so. Adding or removing the seventh does not really change the chord, at least not in my mind. It just defines the harmony more rigidly. Just like the third. With power chords (open fifths) you have a possibility to chose from minor and major thirds. Triads and sevenths define the notes you can use more rigidly. Having said that, in context, most of the time even power chords imply a certain triad (or even a seventh).– cyco130Jun 6, 2014 at 2:37
On the other hand, going from I to I7 (as in V7/IV) does certainly change its function. Hmm, needs more consideration, the whole point of substitution is changing the chord without changing its function.– cyco130Jun 6, 2014 at 2:39
However, I7 would not be diatonic, since it contains b7. The diatonic IM7, OTOH, does not change function. Jun 6, 2014 at 3:00