Have a look at
http://guitarforbeginners.com/extchords.html ...
There are two different 7th notes:
The MAJOR SEVEN is the note one fret below the root (I find it easier to measure DOWN from the root rather than up. The root note is considered as 8, so 7 is just below 8).
The 7th, or flat 7, or dominant 7th (all the same thing) is one fret lower again. Iff you add this note to a minor chord, then it's a minor 7th.
You can also have a chord called minor/major seventh. It uses the major seven note in a minor chord. Sounds silly, bit it's true.
Examples of A chords (Bass string to treble string, numbers indicate frets, x means don't play, 0 means open string):
Plain old A ....X 0 2 2 2 0
A Maj7 ....... X 0 2 1 2 0
A 7th .......... X 0 2 0 2 0
Plain old Am. X 0 2 2 1 0
Am7th ........ X 0 2 0 1 0
AmMaj7 ...... X 0 2 1 1 0
I hope this helps a bit.
Kirk
