Finding a Chord in a key (Diatonic Chord Sequence)

What chords are in what key, and why? This lesson assumes basic knowledge of the Circle of 5ths.
Part 1: Basic Triads.
Each diatonic scale has 7 different notes, which gives way to 7 possible triads for each key in music.

A triad is the 1st, 3rd, and 5th notes of a scale played simultaneously to form a chord.
All chords are formed based on their respective major diatonic scale. A C chord is built on a C major scale, a D chord is built on a D major scale, etc.

When these triads are formed from each degree of the scale we get a series of chords, called the diatonic chord sequence.

There are 7 chords for each key, which correspond to the 7 notes in each key's scale. Some chords can be in more than one key - for example, a D major chord can be in the keys D, A, or G.

Even more is revealed when you look at diatonic quadad (4 note chords) sequence. You should know the formations of Major 7, minor 7, Dominant 7 and min7b5 chords.

I'll use the key of C as an example:
The key of C includes the notes C D E F G A B C.
Each note of the scale corresponds to a scale degree as shown:
Note:     C D E F G A B C
Degree: 1  2 3 4  5  6 7 1

You can form 7 basic chords (triads) from the notes in the key of C. Each different note is the root of a different chord.

We know that triad formation is:
Major triad: 1 3  5
Minor triad: 1 b3 5
Diminished triad: 1 b3 b5

Quadad formation
Major 7th:            1  3 5  7 - Abbreviation: maj7
Minor 7th:            1 b3 5 b7 - Abbreviation: min7
Dominant 7th:       1  3 5 b7 - Abbreviation: 7, dom7
Minor/Major 7th: 1 b3 5  7 - Abbreviation: min/maj7

Your first chord will be a C chord, because C is the first scale degree. Now, since this is a C chord, it will be based on the C major diatonic scale. Take scale degrees 1 3 5 as shown below:
C scale
Note:     C D E F G A B C
Degree: 1   2  3 4 5  6 7 1


This gives you notes C, E, and G. Since all 3 of those notes are in the key of C, you do not have to modify them to fit, and you have a major triad (1 3 5). So your first chord is C major. Similarily quadad is CEGB.

The second chord will be a D chord, because D is the 2nd scale degree. It's based on the D scale, which is D E F# G A B C# D. Now, take 1 3 5 of D scale:
D scale
Note:     D E F# G A B C# D
Degree: 1  2  3   4  5  6  7  1


This gives notes D F# A. This presents a problem - F# is not in the key of C! In order to keep this chord in key, we have to flat the F# (lower it by 1/2 step) down to F natural. This gives D F A, which is scale degrees 1 b3 5 of the D major scale. 1 b3 5 is the formula for a minor triad. Therefore, your second chord is D minor.
 Quadad has notes DF#AC#....so we have to flatten 3rd and 5th note....So it is 1 b3 5 b7......This gives us minor 7th chord....

Our 5th chord is a G chord - let's find the 7th.
Note:    G A B C D E F# G
Degree: 1  2 3  4 5  6 7   1


Our notes are G B D F#. F# (7th) must be flatted to an F natural (b7). Our scale degrees are 1 3 5 b7, which is the formula for a dominant 7th chord. Our 5th chord is G7!

The seventh chord will be a B chord, because B is the 7th scale degree. It's based on the B scale, which is B C# D# E F# G# A# B. Now, take 1 3 5 of this scale:
B scale
Note:    B C# D# E F# G# A# B
Degree: 1 2  3  4 5  6  7  1


This gives notes B D# F#. D# (3) and F# (5) are not in the key of C, and must be flatted to D (b3) and F (b5), respectively. This gives us scale degrees 1 b3 b5, which is the formula for a diminished triad.
Based on these examples, you can figure out the rest of the chords. However, they always follow a pattern:
1 - major
2 - minor
3 - minor
4 - major
5 - major
6 - minor
7 - diminished
In other way
Major … Minor(2)….Major(2)….minor…Diminished(Note alternate maj n minor n end with d)
By applying this pattern, you can quickly figure out that the chords in the key of C are:

Cmaj
Dmin
Emin
Fmaj
Gmaj
Amin
Bdim

All the notes contained in the above chords will be in the key of C.

Shared notes are numbered 1 2 3 4 5 6 7 C  1 3 5 dm 2 4 em 3 5 F 1 4 G 2 5 am 1 3 Bdim 2 4
Notes in the chord
C - E - G         C Major Chord (I) D - F - A          D minor Chord (ii) E - G - B         E minor Chord (iii) F - A - C          F Major Chord (IV) G - B - D        G Major Chord (V) A - C - E         A minor Chord (vi) B - D - F         B diminished Chord (viiº)

This pattern works for any of the keys in the Circle of 5ths. It does not, however, cover any scales that are not the major scale (such as the harmonic minor scale, for example. That has its own pattern of chords).
Chord progressions for various Keys


Using the same method you can figure out the other chords. They also follow a pattern. That pattern goes as follows:
1 - maj7
2 - min7
3 - min7
4 - maj7
5 - dom7
6 - min7
7 - min7(b5)

And, as you may have guessed by now, the chords in the key of C are:
Cmaj7
Dmin7
Emin7
Fmaj7
G7 OR Gdom7 (they are the same chord)
Amin7
Bmin7(b5)

Summary
Lets look at the diatonic chords as Triads and Quadads...

Degree	Triad Notes	Chord	Quadad Notes	Chord
I	C E G =	C	C E G B =	C Maj 7
II	D F A =	D min	D F A C =	D min 7
III	E G B =	E min	E G B D =	E min 7
IV	F A C =	F	F A C E =	F Maj 7
V	G B D =	G	G B D F =	G 7
VI	A C E =	A min	A C E G =	A min 7
VII	B D F =	B dim	B F D A =	B min 7 b5

I hope you know that to improvise over any of the above chords (in any order) you can play the C Major Scale.
Be sure to notice that the order of the chords will stay the same in every key. So you can place this chord sequence on the notes of any major scale. Please check this out yourself and try it on a few different keys, maybe Key of E and Key of Bb. Check that the notes for each chord add up to the chords shown in the sequences below. This will help you memorise the Diatonic Sequences and also help solidify your understanding of chord construction. Both are essential to understand modes.

	I	II	III	IV	V	VI	VII
Triads	Major	minor	minor	Major	Major	minor	dim
Quadads	Maj 7	min 7	min 7	Maj 7	7	min 7	min7 b5

Before you even think about modes you need to be good at playing over these chords in any order, and your ear needs to be tuned into hearing when you are on a good note and when you are on a bad note. Pick a key, write out the diatonic chords in that key and then record them and practice doing solos over them. Or get a jam buddy, get him to play any random chords from a chosen key (he MUST stay only playing chords in that chosen key) and then you solo over it. Then swap over and give him a turn at soloing. Of course this will help you with your barre chords too!
Generally a good note will be a chord tones of any note you are playing over. The bad ones will be the notes in between. Get your ears to recognise the good notes. There is theory behind it of course, and we will explain all that, but most importantly is that your ear can hear the difference between the good and bad notes.
Remember music is about listening, not thinking!

Now we can go to harmonizing the major chord.