7ths and 9ths when should they be used

[l_ricks]

I was just wondering is there a rule of thumb when creating music that applies to using 7ths, 9ths, and other alterations of chords, when creating music?

[comme_le_vent]

no rules

these are mostly for colour - and should be applied according to taste

[BoliverAllmon]

just play what sounds right. Remember that those chords add color but can be more dissonant, therefore you will need a break from them from time to time.

boliver

[Daevren]

They are for the transcended people that can't stand plain triadic harmony.

Ok, this is from the perspective of jaz or modern classical.

The seventh is not a real dissonant. Of course it sounds dissonant but it is a temporary dissonance. It is an imporant part of the harmonic structure. In jazz the main notes are the 3rd and the 7th.

The 9th is a note that is resolved. It is a dissonance, but this one really resolves while the 7th doesn't(it does in alot of music with less dissonant/colourful harmony).

You don't need a break from them. Actually, if you have 6 seventh chords and then have a tonic triad than that is going to sound awkward. Now I personally don't like the seventh tonic chord so I use the 6th chord instead.

If 7th and 9ths are dissonant treat them with the standard rules of treating dissonances.

And yes, 'dissonance' is 'colour'. The more different notes used at the same time, the more intervals. A triad has 3 notes and 3 intervals, a seventh chord has 4 notes and 6 intervals, a ninth has 5 notes and 10 intervals. A eleventh chord has 6 notes and 30 intervals...

Schoenberg used a 11 note chord. That chord has 55 intervals inside it, giving it colour. Its like 55 kinds of paint(although alot of those intervals are going to be equal to others, so the colour analogy is kind of weak in that aspect).