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7ths (Major, Minor or diminished)

7th's are a common Ex-tension of the Chords. With the basic Chord (Major, Minor or diminished), There are only Three Notes in the basic Chord. When you Ex-tend a chord, Then you will have Four notes.

Whenever you Build a Chord, You use Three of the Seven Notes that is considered the basic Chord and the other Four notes are used for the Ex-tension. The 13th's are not studied because They can actually be studied as a 7th. (see Ex-tensions)

All of the Studies at Guitarweek Generally look at the Key of C for Simplicity. So we will use the Key of C again, The Key of C uses the only the Notes (C, D, E, F, G, A, B). Now look what it looks like when the notes are continued:

C D E F G A B C D E F G A B C D E F G A B C
     No matter what key you study this lesson in, Just continue the Seven notes that are used for that Key. The basic Chord use's Three Notes in a Row - above we colored the notes for the C Major Chord. NOTICE that the notes are Every Other One. No Matter which chord you use - the Pattern for the Basic Chord is Every Other One (three notes)

Now Below We have colored in the 7th of C major, As you can see the Pattern is Every Other Note until you have Four notes.
C D E F G A B C D E F G A B C D E F G A B C
Notice Now! the notes that you do Not use for the Major 7th. You use all of the notes for the Key of C EXCEPT for D, F, and A. The notes not used are the Remaining Extensions (9th, 11th and 13th)
Play the C major Chord (CEG) - Then just substitute one of the Notes for a B note, Then you are playing 4 Tones together instead of 3 that makes up the basic Chord. 
Now we will look at how the Pattern applies to the Other Six Chords in the Key of C.

We have colored the Basic chord notes Blue ---- and the Ex-tended note red

C Major 7th C D E F G A B C D E F G A B
D Minor 7th  C D E F G A B C D E F G A B C  
E Minor 7th  C D E F G A B C D E F G A B C D
F Major 7th  C D E F G A B C D E F G A B C D E
G Major 7th  C D E F G A B C D E F G A B C D E F  
A Minor 7th  C D E F G A B C D E F G A B C D E F G  
              B Dim.        C D E F G A B C D E F G A B C D E F G A     

This is a Mathematical Lesson - Notice The PATTERN that we used above for the Ex-tension's. It  is Every Other Note that is in that Key.
If you LOOK close You should Realize that the basic Chord is the 1st, 3rd and 5th note from the starting point - and the ADDED 7th is the 7th note from the starting point - and the ADDED 9th is the 9th note from the starting point - and yes the 11th is the 11th note from the starting point!