C Major Scale

C Major Scale

To review, when creating chords, we usually follow the concept of stacking thirds which means that you skip every other note in the scale. For instance, when creating the C Major triad, we start with the root note, C, then skip the next note in the scale, D, and use the third note, E. Then skip a note, F, and incorporate the next note, G. This results in the C Major triad with the notes C, E, and G.

C Major vs CM7

Again, the notes that make up the C Major chord are C, E and G.
In order to make the CM7 chord, we need to continue stacking thirds.
After the note G in the scale, comes A.
We will skip this note and add the next note, B, to our chord.
So the notes of the CM7 chord are C, E, G and B.
We call this a major 7 chord because the triad is a major triad and the interval between C and B is a major 7 interval.

To make the CM7 guitar chord, we need to incorporate the note B into our chord.
This is easily done by lifting off our first finger, first fret, second string which sounds the note C.
By playing this string open, we have added the note B to our chord, thus producing the CM7 chord.

Dm vs Dm7

Following our rules for stacking thirds we can figure out the notes for the Dm chord as being D, F, and A.
Continuing for the Dm7 chord, we end up with the notes D, F, A and C.
This is called a minor 7 chord because the triad is a minor triad and the interval between D and the 7th, C, is a minor 7 interval.
To make the Dm7 chord on guitar, C needs to be added. This is done by creating a mini bar on the first two strings in the first fret with the first finger, and then placing the second finger, second fret, third string.

Em vs Em7

The notes that make up the E minor chord are E, G and B. If we continue stacking thirds, we add the note D to produce the Em7 chord.
To create the Em7 guitar chord, form the typical Em chord and then place your fourth finger in the third fret on the second string. This sounds the note D to complete our Em7 chord.

F vs FM7

F Major consists of the notes F, A and C. Continuing to stack thirds to the chord will add the note E thus producing the FM7 chord. This chord consists of the major triad and the interval between F and E is a major 7 leading to the major 7 chord.
In order to produce the FM7 guitar chord, simply lift your first finger off of the first string in the first fret so that it is only fretting the second string. This is changing the note on the first string from F to E.

G vs G7

The notes in the G Major triad are G, B and D. Extending that by stacking thirds results in adding F to the chord.
This produces the G7 or G dominant 7 chord.
This seventh chord is different from both major 7 and minor 7 chords.
It is the result of adding a minor 7 to a major triad. The interval between G and F is a minor 7 interval. This produces the unique sound of the dominant 7th chord.
To produce the G7 guitar chord, we change the note played on the first string. Instead of playing in the third fret which sounds the note G, play the first fret to produce the note F.

Am vs Am7

Am consists of the notes A, C and E.
Am7 extends this triad by adding the note G.
To create the Am7 chord, take the typical Am chord and add the fourth finger to the third fret, second string which is the note G.

Bdim vs Bm7b5

Stacking thirds starting on the seventh note of a major scale produces a three note diminished chord and a four note half diminished seventh chord which is also known as a m7b5 (minor 7 flat 5).
In the key of C, these are B diminished (B, D and F) and Bm7b5 (B, D, F and A) respectively.
We can add A to the B diminished chord by playing A on the third string instead of B.
To create the chord:
First finger, second fret, fifth string
Second finger, third fret, fourth string
Third finger, second fret, third string
Fourth finger, third fret, second string

Recap

So there are the 7th chords in the key of C and how they are created and played on the guitar.

Exercises to practice changing between the triad and seventh versions of these chords can be found at the following links and also employ new strumming patterns so be sure to check them out as well:
C to CM7
Dm to Dm7
Em to Em7
F to FM7
G to G7
Am to Am7