Okay so for about a year and a half now i've been hearing all this chat about altered chords. So I have a few questions:

1.) Whats the formation for a altered chord? (e.g R35 etc.)
2.) Where can they be applied?
3.) And where abouts in a scale are they? (e.g Cmaj is the first chord of Cmaj scale B dim is the last etc.)

Cheers guys, oh and also if you could post a few voicings of lets say C alt. please?

Thanks alot, Aidan.
Find a lesson on the CAGED system, there's probably one on this site.
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I allready know the CAGED system. And thanks, but it really doesn't help any of my questions, any more answers?
I could be totally wrong, but I thought it would be something like either a b5/b9 or #5/#9 chord.

I'm really not sure about it...
I'm going to assume here you mean altered dominant chord, and not any chord with an alteration, and just a little preface to avoid confusion: There is a difference between an altered chord and a chord labelled _alt, even though a chord labelled _alt is still an altered chord. That is to say, a chord labelled _alt is an altered chord built from a specific scale, the altered dominant scale (7th mode of melodic minor). You wouldn't call every dominant chord with an alteration _alt.

Quote by bass wizard
1.) Whats the formation for a altered chord? (e.g R35 etc.)

I'll start with giving you the formation of the altered dominant scale: 1 - b2 - #2 - 3 - b5 - #5 - b7. (Note here that you can name the b5 and #5 interchangeably with #11 and b13 if it causes you any confusion. Infact the #5 of the altered dominant as I've written it is usually referred to as a b13, because this avoids confusing a 7#5 chord built from this scale with one built from the whole tone scale). So any chord built from this scale can be given the label _alt, and be considered an altered dominant chord. Anything from 7b9 (1 - 3 - b7 - b9) to 7b5#5b9#9 (1 - 3 - b5 - #5 - b7 - b9 - #9; all the tones the scale contains). Obviously you can use any combination of the alterations in the scale (b9, #9, b5 and #5) to form an _alt chord.

Chords like 1 - 3 - 5 - b7 - b9 are still altered chords, this one is a 7b9. But it isn't built from the altered dominant scale, as you can see this chord has a perfect 5th, our scale doesn't. This is what I meant in my preface.

Take all the chord tones from a straight up dominant, with and without the 5th (1 - 3 - 5 - b7 and 1 - 3 - b7) and then you can pick and choose different combinations of alterations to see what you like the sound of.

Quote by bass wizard
2.) Where can they be applied?

Anywhere you'd use an unaltered dominant chord. That said, given we are altering and adding tones, each different alteration and combinations of, tend to want to resolve to different places. An example might be 7b9 wants to resolve to a minor chord a perfect fifth down (resolving here to a major chord is fine too; the reason I say minor is because the 7b9 is most commonly built from harmonic minor. Thus a resolution down a fifth is usually to the tonic minor chord), where as if you play a 7#11 chord, your ears might prefer a resolution down a perfect 4th. (Down a half step and up a whole step are also commmon resolutions for this chord. Also if you're confused at any point as to the naming of enharmonic resolutions: always name the alteration from the scale your chord is built from; i.e. do not call a 7#11 chord built from Lydian Dominant as 7b5). An example of application of this chord might be to alter a standard _7 chord to include the #11 in a tritone substitution because of its tendency to resolve down a half step.

Here I'd just recommend you do as above, and mess around with them, see where you think specific chords do or do not fit. Just try taking a song or progression you know with loads of dominant chords in it, and give them alterations.

Quote by bass wizard
3.) And where abouts in a scale are they? (e.g Cmaj is the first chord of Cmaj scale B dim is the last etc.)

I'd recommend you endeavour to find out this information yourself as you come to digest the concept of them. Start with simple things like 7b9, 7#9 and 7#11 and as you come to understand their application and sound, you might like to then go on and understand from which scales they can be built. I don't think this information should be your first concern, and I recommend you attempt to 'make your own' voicings of these chords; I believe feeding them to you on a plate is counterproductive, so I won't be including any specific voicings in this post.

I will however, give you the formulae and notes in C that are contained in the three chords I recommend you start with.

7b9: 1 - 3 - (5) - b7 - b9: C - E - (G) - Bb - Db.
7#9: 1 - 3 - (5) - b7 - #9: C - E - (G) - Bb - D#.
7#11: 1 - 3 - (5) - b7 - (9) - #11: C - E - (G) - Bb - (D) - F#.
Altered chords are mostly used as substitutions for regular diatonic chords. That's the simplest way to approach them. Since they are altered, they won't fit into the diatonic scale...so you get altered scales or just a different scale. I think that the word "altered" may be used differently in classical and contemporary music. But the premise is the same. Taking a chord and moving one(or more) chord tones a semitone. Most commonly the 5th and the 9th. Like in Jazz you might call it a parallel secondary dominant.

If you want to figure out the scale used by say D7(b9) in a BbMaj progression, take your Phrygian scale and alter it by "forcing" the chord tones of D7(b9) into it. What you end up with will be G Harmonic Minor or more specifically Mode 5 Harmonic Minor*cuz you started the scale on D*. The reason in theory that Mode 5 Harmonic Minor will work over a tune in Bb Major is that the key tonality of a tune will always be stronger then the tonality of an altered chord. Since all you did was alter the notes of a diatonic scale and force the notes of chord into it, what you get is scale that is still close to the tonic Bb which gives you ample room for resolution of tension.

As far as what formula to use for the chord, it depends on the progression. In my example, the formula for D7(b9) in a Bb progression would be 1, #3, 5, 7, 9 with the 3rd being altered.

Reading now what I typed, I think that may only partly answer your question...haha, but maybe it'll help.

EDIT: I hate it when I'm writing a long post and someone beats me to it. lol. Nice post JohnlJones7443
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Alt chord is loosely defined as a chord with both an altered 5 and an altered 9

It could be #5 and #9 or b5 and b9

Ive seen the sharp configuration more often then the flat