Rather than finding a web site (- I rarely have an internet connection when I'm playing with people :)!) for a small amount of pain get the gain from spending a short time understanding how to form chords and you can then work out any chord anywhere ALL THIS WORKS IN ANY KEY so once you have the principle you can pretty much work any chord out.

This is probably a dangerous thing to attempt but what the hell. I was going to do this for a friend to explain about making chords so it has just brought it forward. I'd be interested to know if it's understandable and/or useful. (There are probably lots of holes that can be picked but any comments welcome.)

BASICS OF UNDERSTANDING HOW TO MAKE CHORDS

I assume you know which notes are which on a guitar so you know that the chord you asked for contains three A's a C# and an E. If that isn't obvious, ask. This would work for any instrument that is worked in semitones - so most stringed instruments that you come across commonly (not a dulcimer which doesn't go up in semitones!) and most keyboard instruments where each note going upwards is one interval (a semitone) higher than the previous

  1  2   3  4  5   6  7   8  9  10  11 12  1  2   3  4  5   6  7   8  9  10  11 12
  
  A  A#  B  C  C#  D  D#  E  F  F#  G  G#  A  A#  B  C  C#  D  D#  E  F  F#  G  G#
     Bb        Db     Eb        Gb     Ab     Bb        Db     Eb        Gb     Ab  
  

(I've repeated the notes and numbers to mean you won't have to go back round!)

MAJOR SCALES have the following intervals in them - root note - root+2 - root+4 - root+5 - root+7 - root+9 - root+11 - root note again (ie root+12). EVERY scale is the same so as an example C has the following notes - C root note - D root+2 - E root+4 - F root+5 - G root+7 - A root+9 - B root+11 - C root note again ; Bb has the following notes - Bb root note - C root+2 - D root+4 - Eb root+5 - F root+7 - G root+9 - A root+11 - Bb root note again

So far so good?

MAJOR CHORDS are built from the root of the chord (if you play the x02225 chord as a whole and listen to it and had to sum up the overall sound most people would choose the Open A string I would guess as the dominant overall sound of the chord - most guitar chords usually have this as the lowest string played - eg A - x02220 C - x32010 D - xx0232 E - 022100 G - 320003 etc)

A major chord will then have two additional notes - the first one which is four intervals (semitones) higher (and which is the third note of the major scale - in Do-Re-Mi it's the Mi note) and the second one is seven intervals (semitones ) higher than the root note (the Soh note in Do-Re-Mi which is the fifth note of the scale). This works for all major chords.
Because a guitar has more than three strings some of the notes will be repeated - in the original example the A is repeated twice and the bottom string ignored. Usually the root note is the most likely to be repeated.

On my primitive little diagram above. Start with a note above - D# for example. Count 4 intervals to the right which gives a 'G' and then count a further three intervals to the right which gives an A# and that is a D# major. (You would never mix sharps and flats in the same chord so Eb-G-A# or D#-G-Bb are not OK!)

Given a quick bit of working out you'd soon get the following for the major chords:

A - A C# E A
Bb - Bb D Eb Bb (A# - A# D D# A#)
B - B D# F# B
C - C E G C
C# - C# E# G# (I know that ones weird but you can't conventionally have an F and an F# in the same scale so what we normally refer to as an F is called a sharpened E - this only overcomplicates things, the number of intervals between the notes is constant)
etc

Hopefully you could work the rest out by now if this has made sense.

SOME OTHER MAJOR CHORDS

SEVENTH CHORDS In the orginal post A7 was mentioned. To form these you create your major chord (eg A C# E A) but rather than the top note being 12 intervals from the root you choose the one that is 10 intervals away - so an A7 has A C# E and G; F would be (major chord F A C F) F A C Eb etc etc Sevenths contain the flattened (ie one interval to the left) seventh note of the major scale (in case of A - A B C# D E F# G# A hence G)

NINTH CHORDS To form these you take the seventh you have created and add the note that is 14 intervals above the orginal - so an A9 has A C# E G and B; F would be (major chord F A C F) F A C Eb and G etc etc (For ease take 12 off the 14 intervals as it's easier to work out the ninth note is the same note as the one 2 above the root note)

ELEVENTH CHORDS To form these you take the ninth you created earlier and add the note that is 17 intervals above the orginal - so an A11 has A C# E G B and D; F would be (major chord F A C F) F A C Eb G Bb etc etc (For ease take 12 off the 17 intervals as it's easier to work out the eleventh is the same note as the one 5 intervals above the root note)

THIRTEENTH CHORDS To form these you take the eleventh you have created and add the note that is 21 intervals above the orginal - so an A9 has A C# E G B D and F#; F would be (major chord F A C F) F A C Eb G Bb and D etc etc (For ease take 12 off the 21 intervals as it's easier to work out the thirteenth is the same note as the one 9 above the root note)

MINOR CHORDS

The note that defines a minor chord is the third note of the scale. In the key of C rather than the E of the major chord the note for a minor chord is Eb (three intervals/semitones from the root of the key rather than the four of the major).

So if you substitute a flattened third on all of the chords above then you well get the minor equivalent. For example, Am9 - A C E G B; Fm7 - F Ab C Eb;


A FEW OTHER CHORD TYPES -

MAJOR SEVENTH CHORDS (eg Amaj7) To form these you create your major chord (eg A C# E A) but rather than the top note being 12 intervals from the root you choose the one that is 11 intervals away - so an Amaj7 has A C# E and G# ; F would be (major chord F A C F) F A C E etc etc. Major sevenths contain the seventh note of the major scale (in case of A - A B C# D E F# G# A)

MAJOR NINTH CHORDS To form these you take the major seventh you have created and add the note that is 14 intervals above the orginal - so an A9 has A C# E G# and B; F would be (major chord F A C F) F A C E and G etc etc (For ease take 12 off the 14 intervals as it's easier to work out the ninth is the same note as the one 2 above the root note)

sus 4 CHORDS To form these you take the root note - root+5 - root+7 so a Gsus4 has a G C D; an C#sus4 has a C# F# G#

SIXTH CHORDS To form these you take the add the sixth note of the scale (9 intervals higher than the root) - so an C6 has C E A; F would be (major chord F A C F) F A D etc etc Sometimes I include the 5th note also sometimes not - I think technically these are 'added sixths'

And that's pretty much it. Occasionally you'll see chords like G13b9 - all this means is that it is a G13 ( G B D F A C E but with the ninth note moved one to the left - G# rather than A - so it has G B D F G# C E - you have to leave at least one out though unless you have a seven string guitar and are polydactyl)

Do diminished and augmented chords too if this has been useful.

Once you understand this it's easy working out any chord.

Try D13b5b9 - if you can work that out you understand it and never need another chord book ever.