Where do extended chord belong?

Question

If I wanna write in the key of A major, can I use a C9? Or is it I have to make chords with notes from that scale of the key I use?

Answer 1

That's your choice to make. C7, C9, C11, C13. Experiment and decide which sounds best to you.

Answer 2

You can use "accidentals" (sharps, flats, and naturals) to make any chord you want.  The question is will it be something people will want to listen to.

Answer 3

@Mordent

I've never been to America.

Answer 4

Strictly speaking you can ONLY use 7 chords (and limited extensions of those chords) within a key. In A that would be

A Bm C#m D E F#m G#ø

Some examples of extensions of A that you could use are A6 A^ A2 Asus4 and so on. Any note you can add to a chord that is in the scale of A. You can't use A7 because G isn't in A major. You can't use Dm because you don't have an F to use. You can't use C9 because you don't have a C or G or Bb.

ANYTHING else is not "in the key" - but chords outside this original key are used very frequently, for the simple reason that writing things in one key all the time is very boring. It is the musical equivalent of someone never travelling out of Wyoming. Even in periods where there were a lot of rules to follow (such as the Baroque or Classical periods) taking small diversions out of the original key, or changing key altogether were VERY common. Staying in one place hasn't really been common since the medieval period (in western music anyway).

That being said there are more common destinations than others. In A the chord of C#7 is commonly heard (because it is in the relative minor - F#m). Think of that like New York for your Wyoming resident. A lot of people will have been there, it's not very surprising to hear someone travel there. A more exotic chord would be something like Bb7 - (it is similar to the dominant E7). This is like Paris - quite a leap and notable, but not THAT crazy. Something like Gm? That is pretty weird if you're in A - it has nothing in common with A major. It's like someone travelling to Pakistan - it is definitely going to get people talking, and some people might want you to justify why you're going there!

Answer 5

You have two questions:

Extended chords, basically, belong wherever the composer, arranger or performer wants to add more colour or a more complex harmony that they'd get with basic triads.

Chords that are made from the notes in the scale related to the key you are using are called diatonic chords. Chords including notes outside that scale are called chromatic chords. You can use whatever chords sound best and provide the accompaniment you want.

Answer 6

You can do anything that sounds good to you.

Just try learning songs by other people and learn from those rather worrying about rules.